Base Station Base Methods and  Apparatus For Supporting Break Before Making Handoffs In A Multi-Carrier System

ABSTRACT

A mobile communications device initiates a handoff from its current base station (BS) sector network attachment point to a new BS sector. The mobile sends a handoff request over its current wireless link to the current BS sector, which forwards the request to the new BS sector, e.g., via a network link. The new BS sector processes the request assigning dedicated resources, e.g., an identifier and dedicated uplink segments. Information identifying the allocated resources is conveyed from the new BS sector via the current BS sector to the mobile. The mobile determines the time of the allocated dedicated segments based upon a received beacon signal from the new BS sector with known timing relationships to dedicated segments. The mobile breaks the original wireless link just prior to the time of the first assigned dedicated segment. The mobile communicates information on the assigned dedicated segments to perform registration operations, e.g., timing synchronization and power control, establishing a new wireless link.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 11/283,676, filed Nov. 21, 2005 which application is acontinuation of U.S. patent application Ser. No. 10/964,945 filed Oct.14, 2004, now U.S. Pat. No. 7,047,009, which claims the benefit of U.S.Provisional Patent Application Ser. No. 60/527,475 filed Dec. 5, 2003,for which all are hereby expressly incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to multi-carrier communications systemsand, more particularly, to method and apparatus for performinginter-sector and/or inter-cell handoffs in such systems.

BACKGROUND

Cells may include one or more sectors. A cell without multiple sectorsis a single sector cell, i.e., it includes a single sector. Signals arenormally transmitted by a sector transmitter using a carrier frequencyand the corresponding bandwidth, e.g., one or more tones surrounding thecarrier frequency. Different cells and/or sectors of a cell often usedifferent frequency bands centered around a carrier frequency used bythe sector or cell. The carrier frequency of adjacent cells and/orsectors are often different. To receive signals corresponding to acarrier frequency, a wireless terminal normally has to adjust itsreceiver, e.g., receiver filters, to correspond to the frequency bandassociated with the carrier frequency to be used. Switching a receiverbetween carrier frequencies may take time. Thus, in receivers with asingle filter chain, transitioning between different carriers may causethe receiver to encounter intervals during which information can not bereceived due to the switching process.

Wireless terminals, e.g., mobile nodes, communicating with a basestation on a given carrier frequency and moving through a multi-carriersystem need to decide when to make a handoff and transition to a newcarrier frequency, e.g., corresponding to a new cell and/or sector. Asdiscussed above, an adjacent sector and/or cell may use a differentcarrier frequency, and as a sector or cell boundary is crossed, awireless terminal will normally have to identify and switch to the newcarrier frequency.

Typically a mobile node includes a single receiver chain and listens toone carrier frequency band at a given time due to constraints in thehardware and cost associated with the receiver. This is because, forcost reasons, multiple parallel receiver filter chains are often tooexpensive to be practical. In some known systems a mobile node waitsuntil communications are lost or significantly degraded on the operatingcarrier band being used before switching to another carrier. In somesystems, a wireless terminal periodically switches its receiver to adifferent carrier band to check for signal presence and/or strength.Unfortunately, while switched to search for another carrier, thereceiver can not receive signals from the carrier that is currently inuse. The known methods of determining what carriers are available toswitch to and when to switch to a new carrier may result in interruptedcommunications, gaps during the hand-off process, and/or wastedresources in monitoring and determining the appropriate carrierfrequency band.

In addition to the problem of determining which carriers/frequency bandsare available and should be used at any given time, handoffs betweensectors and/or cells using different carriers present problemsassociated with adjusting receiver and/or transmitter circuitry toswitch between carrier frequencies. Problems associated with switchingbetween carrier frequencies occur when a switch between carriers occurswhether or not a change in location occurs and are generally encounteredwhen handoffs occur between carrier frequencies. For cost reasons, it isoften desirable to implement a communications device with a singlereceiver and transmitter.

When switching between carrier frequencies, an analog filter used by thereceiver and an analog filter used by the transmitter normally has to bechanged to match the new frequency band. This normally involvesadjusting the filter as a function of the carrier frequency of the newsector or cell. The transitional period required to implement thisfilter change, in the case of a device with a singlereceiver/transmitter results in an interval during which thecommunications device is unable to receive and/or transmit informationto a base station.

In systems where each cell/sector uses the same frequency, e.g., insystems with a frequency reuse rate of 1, handoffs between sectorsand/or cells do not require such filter switching operations since thefrequency band used in each of the sectors/cells is the same. In suchsystems “make before break” handoffs are relatively easy to implement.In a make before break handoff the communications device directlycommunicates with the new sector and/or cell before breaking, e.g.,terminating, the connection with the old base station. Given that thecarrier frequencies are the same before and after handoff in suchsystems, there is normally no need to alter the filters in the receiverand/or transmitter circuitry making the time required to switch betweenthe two sectors and/or cells relatively minor.

Regardless of whether a handoff operation involves a change in carrierfrequency or not, in many systems when handing off from one base stationor sector to another before a mobile device is permitted to transmituser data, e.g., application layer data such as voice, text, etc., themobile node performs timing and/or power control synchronizationoperations. Registration in the sector or cell being entered is normallyalso required before transmission of user data to the new base stationor sector is permitted. Such signal level synchronization operations canbe important to prevent transmission by the mobile device entering thecell and/or sector interfering with the transmissions from other mobiledevices already in the cell/sector being entered. In some systems, aparticular period of time is set aside on a periodic basis for use bymobile devices entering the system to transmit signals used to registerand/or perform initial timing and/or power control synchronizationoperations. During such periods of time, devices entering thecell/sector can contact the base station to perform timing and/or powercontrol synchronization operations without interfering with devicesalready in the system, e.g., because registered devices know not totransmit signals during this particular period of time. Signaling duringthis dedicated period of time is often contention based, e.g., one ormore new devices may attempt to register using the same communicationsresource, e.g., set of tones. In such cases, signals may collide and theregistration by the devices attempting to use the same set of tones mayfail requiring them to retry during a later dedicated registrationperiod, e.g., using another set of tones. As part of the registrationprocess, physical layer signaling issues are resolved such as physicalsignal timing used to control symbol transmission and/or transmissionpower control is achieved, e.g., based on control signals received fromthe new base station. In addition, one or more device identifiers usedto identify the device while in the new cell may be assigned to thedevice seeking to register in the new cell/sector. Once synchronizationand ID assignment issues are resolved in regard to the new cell/sector,higher level signaling, e.g., IP packet transmission and reception maybegin to occur between the mobile device entering the new sector and/orcell and the base station in that sector/cell.

In the case where the frequency bands of the old and new sector and/orcell are the same, it is often possible to maintain communications withthe old base station while simultaneously communicating in the samefrequency band with new base station to perform the above discussedregistration operations, e.g., timing control, power control andcell/sector ID specific assignment operations. This is possible sincethe frequency of the filter used in the receiver and/or transmitter neednot be changed when communicating with base stations in cases where theold and new carrier frequencies are the same. Thus, in systems where theold and new frequency bands are the same a mobile device can completephysical layer signaling operations which need to be completed before IPpackets can be received/transmitted in the new cell while still beingable to receive IP packets from the old base station. Once the physicallayer, e.g., timing synchronization, etc., with the new base station,and other registration operations are completed in the new sector/cell asignal may be sent to trigger re-routing of IP packets to the mobiledevice by way of the new sector/cell and to stop the routing of packetsintended for the mobile to the old sector/cell. In this way, in variousknown systems, the connection with the old cell is broken after aconnection, sufficient to communicate IP packets, with a new cell orsector, is established.

While using a single carrier which is the same in each sector and cellof a system simplifies handoff operations, it has drawbacks due to therelatively high degree of interference at sector and cell boundaries. Atsuch boundaries, given signal fading, mobile nodes may experience signalconditions considerably worse than 0 dB for extended periods of time.

When different sets of frequencies are used in adjoining sectors/cells,e.g., a frequency reuse pattern greater than 1 is employed, signalconditions at sector and cell boundaries are usually considerably betterthan in cases where there is full reuse of all frequencies. Thus, signalinterference at cell/sector boundaries provides a reason to avoid afrequency reuse scheme of 1 despite the handoff benefits it provides.

Delays associated with adjusting a transmitter and/or receiver's filterto operate at a new frequency band makes switching receiver andtransmitter circuitry between an old and a new carrier frequency at arate that is quick enough to support the above discussed make beforebreak handoff procedure difficult to implement. Accordingly, in handoffsbetween sectors and/or cells using different frequency bands, a breakbefore make handoff operation is often used where radio signaling withthe old base station is terminated before it is established with a newbase station. Unfortunately, this normally leaves the mobile deviceunable to receive IP packets not only during the duration that it isswitching its filter circuitry to the new carrier frequency but for theadditional time period it needs to register with the new cell/sector andto perform the required timing and/or power synchronization operation(s)and any IP packet redirection operations that may be needed.

The need to wait, in some systems, for a periodically occurring timeperiod during which registrations are permitted to occur within a sectoror cell, combined with the uncertainty that resources will be availablein the cell or sector for the mobile device to register during aparticular registration period, can lead to both unpredictable andsometimes excessive delays before a mobile device can receive IP packetsin a new cell or sector after terminating a connection with an old basestation.

In view of the above discussion, it should be apparent that there is aneed for methods and apparatus for reducing the amount of time requiredto complete a handoff in a system which uses different frequency bands.It is desirable that at least one or more methods be provided whichavoid the need for a mobile device to terminate a connection with acurrent base station and/or cell before it can commence communicationwith a new base station or cell in regard to handoff related matters,e.g., registration signaling, assignment of airlink related resourcessuch as local identifier assignments, etc. It is also desirable, that inat least some embodiments, that a mobile device be able to expect with areasonably high degree of certainty the communications resources neededto complete a registration process will be available at or near the timeit terminates communication with a previous base station.

SUMMARY OF THE INVENTION

The present invention is directed to methods and apparatus for switchingbetween communications links implemented using different carrierfrequencies, e.g., has part of a handoff between sectors and/or cells oras part of an intra-sector handoff between two different carrierfrequencies used in a sector. The methods of the present invention areparticularly well suited for use where the system supports the use ofdifferent frequencies for communications purposes, e.g., in differentsectors, cells or within a sector.

In a system using the invention, base station transmitters in differentsectors and/or cells periodically transmit a high power signal,sometimes called a beacon signal, into the frequency band used in theneighboring sector or cell. Beacon signals are signals which include oneor more narrow (in terms of frequency) signal components, e.g., signaltones, which are transmitted at relatively high power compared to othersignals such as user data signals. In some embodiments beacon signalseach include one or more signal components where each signal componentcorresponds to a different tone. A beacon signal component in someembodiments includes a per tone signal energy which is 10, 20, 30 ormore times the average per tone signal energy of signal tones used totransmit user data and/or non-beacon control signals.

Multiple beacons, e.g., multiple high power tones can be transmitted atthe same time although in many embodiments at most a single beaconsignal is transmitted by a transmitter in any given transmission timeperiod, e.g., symbol transmission period. The single beacon signal mayinclude a single high power signal tone or, in some embodiments, a fewhigh power tones.

In accordance with the present invention handoff operations areinitiated by a wireless terminal, e.g., a mobile communications device,via a current base station sector with which the mobile device has awireless communication connection, e.g., a first communications linkimplemented using a first carrier frequency. The mobile devicecommunicates via the first communications link and the current basestation sector its desire to complete a handoff to a different basestation, sector, or carrier within the sector in which the mobile deviceis located. The new communications link will be established using a newcarrier frequency which will often be different from the first carrierfrequency. The base station sector with which the new communicationslink is to be established, referred to as the new base station sector,assigns to the mobile device, via the current base station sector andthe first communications link, one or more airlink related resources tobe used upon entry into the new base station sector or upon switching tothe new carrier frequency within the current sector when the new basestation sector is the same as the current base station sector. Theairlink related resources may be one or more device identifiers (such asMAC state identifiers, e.g., ON state identifier, ACTIVE stateidentifier) which are to be used when communicating in the new basestation sector using the new carrier frequency. As part of the handoffprocess the new base station sector may dedicate and thereby reservephysical signaling resources, associated with the new carrier frequency,for use by the mobile device initiating the handoff operation, e.g.,dedicated communications bandwidth such as, e.g., a set of tones, to beused to complete the registration process upon entry into the cell usingthe new carrier frequency. The dedicated set of tones may be used, e.g.,for power control and/or timing control operations upon entry into thenew base station sector. Such dedicated resources may be allocatedwithin a periodically occurring access or registration time period. Thenew base station sector communicates, in some embodiments, informationidentifying the particular registration period in which the mobile wasallocated the dedicated resources. This information is used, in variousembodiments, to determine when the current communications link with thecurrent base station sector should be terminated and a newcommunications link using the new carrier should be established with thenew base station sector, so that the disruption of services due to thetermination of the first communications link can be minimized.

After making a decision to initiate a handoff operation, the mobile nodeand/or current base station sector sends an IP routing update message toa mobility agent, e.g., Mobile IP home agent, used to redirect IPpackets intended for the mobile device to the base station sector beingused to attach the mobile device to the network. The IP routing messagecauses the mobility agent to begin redirecting IP packets intended forthe mobile device to the new base station sector to which the handoff isto be completed. In some embodiments, the transmission of the IP routingupdate message is sent after receiving a device identifier to be used inthe new base station sector and/or dedicated resources to be used in thenew base station sector, e.g., to complete a registration process. Thisensures that the new base station sector has resources available toservice the mobile device seeking to complete the handoff to the newbase station sector.

In the above described manner, a mobile node can initiate a handoff to anew base station, sector or carrier within a sector involving a changeto a different carrier frequency via its existing communications link.In this manner, the need to tune to the new carrier frequency to beginthe establishment of a connection using the new carrier frequency can beavoided and the mobile node can receive resource assignmentscorresponding to the new base station, sector and/or carrier frequencywithout having to first switch to the new carrier frequency. Resourcesassigned by the new base station or sector may include, e.g., a sectorspecific and/or sector carrier specific device identifier to be usedwhen communicating in the new sector and/or using the new carrierfrequency. Dedicated communication segments for establishing thecommunication signaling, e.g., power, timing control, and/orregistration signaling, may also be assigned by the new basestationand/or sector with the assignment being communicated over the firstcommunications link to the mobile node before establishing signalingover the new communications link using the new carrier frequency.

In accordance with one feature of the invention, in some embodiments anIP routing message is normally sent after initiation of a handoff to anew basestation, sector or carrier frequency within a sector but priorto the point where the mobile node has completed registration, powercontrol and/or timing control over the communications link beingestablished with the new basestation, sector or carrier frequency. Insuch a case, the IP routing update process will normally be initiated toredirect IP packets to the cell, sector or circuitry within a sectorcorresponding to the new carrier frequency before the mobile node isable to transmit user data over the new communications link beingestablished. Thus, in many cases, the IP routing message is normallysent prior to completion of the handoff, e.g., prior to termination ofthe current communications link in favor of the communications linkbeing established as part of the handoff process. In suchimplementations, IP routing update delays will at least partiallyoverlap the period during which the mobile node is likely to unable tocommunicate with either the old or new base station sector as a resultof the process of changing receiver and/or transmitter circuitry, e.g.,filter circuitry, to correspond to the new carrier frequency to be usedwhen communicating with the new communications link being established aspart of the handoff process.

In the case of single sector cells, handoffs between old and new basestations correspond to handoffs between base stations of different cellsdue to the one to one relationship between cells and base stationsectors. However, in the case of multi-sector cell implementations,intra-cell inter-sector handoffs are possible with the old and newsectors being in the same cell. In some embodiments, in intra-cellinter-sector handoffs, timing synchronization is maintained between thebase station sectors, and the timing synchronization steps, normallyperformed, in a handoff process are omitted. In such cases, a handoff toa new sector of the same cell can be completed without a timingsynchronization operation being performed. Thus, upon entering the newsector, following termination of the old communications link, the mobiledevice can begin transmitting user data prior to receiving a timingsynchronization signal from the base station or performing a timingsynchronization operation. This is because timing synchronizationbetween sectors of the cell is maintained in some embodiments andrelying on the timing synchronization initially achieved in one sectorof a cell is not likely to cause interference problems in the othersynchronized sector of the same cell. Skipping an initial timingsynchronization step, which is normally required upon entry into a newcell, when implementing an intra-cell handoff, reduces delays associatedwith implementing intra-cell handoffs as opposed to inter-cell handoffs.

While the method and apparatus of the present invention still involvebreaking radio communication over an existing communications linkimplemented using a first carrier frequency, prior to establishing radiocommunication using a second, e.g., different, carrier frequency, thesignaling exchanged prior to this operation by way of the existingcommunications link which uses the first carrier frequency allows themobile device to obtain some of the benefits of a make before breakhandoff, e.g., ID assignments and airlink resource allocations, prior toactually breaking communication over an existing link thereby reducingthe latency and uncertainty associated with many make before breakhandoff operations.

Thus, the methods and apparatus of the present invention represent animprovement over older break before make handoff methods in which amobile device would first break an existing link before being able toreceive resource assignments, etc. in regard to a new communicationslink being implemented using a different carrier frequency.

Numerous additional features and benefits of the methods and apparatusof the present invention are discussed in the detailed description whichfollows.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a drawing of an exemplary three sector cell including asectorized base station and a wireless terminal situated on a sectorboundary, said base station and wireless terminal implemented inaccordance with the present invention.

FIG. 2 is a drawing of an exemplary multi-cell multi-sector wirelesscommunications system including three sectorized base stations and awireless terminal situated on a cell boundary, said communicationssystem implemented in accordance with present invention.

FIG. 3 is a drawing showing exemplary downlink signaling from eachsector of an exemplary three sector base station, in accordance with oneexemplary embodiment of the present invention.

FIG. 4 is a drawing showing exemplary downlink signaling from twosectors of the same type designation from different adjacent cells inaccordance with the present invention.

FIG. 5 is a drawing of an exemplary communications system implemented inaccordance with the present invention and using methods of the presentinvention.

FIG. 6 is a drawing of an exemplary access node (base station)implemented in accordance with the present invention and using methodsof the present invention.

FIG. 7 is a drawing of an exemplary wireless terminal (end node)implemented in accordance with the present invention and using methodsof the present invention.

FIG. 8 is a drawing of exemplary downlink beacon signals, exemplaryuplink dedicated segments and contention based uplink segments which canbe used for access purposes, and exemplary timing relationships, and isused to explain various features of the present invention.

FIG. 9 is a drawing of an exemplary system, implemented in accordancewith one exemplary embodiment the present invention, and is used forexplaining various features and signal flows related to handoffoperations in accordance with the present invention.

FIG. 10 is a drawing illustrating exemplary handoff signaling inaccordance with the present invention.

FIG. 11 is a flowchart of an exemplary method of operating a wirelesscommunications system to perform handoffs of a wireless terminal fromone base station sector point of attachment to another base stationsector point of attachment in accordance with the present invention.

FIG. 12 is a flowchart of an exemplary method, in accordance with thepresent invention, of operating a mobile communications device toimplement a handoff of the mobile communications device between a firstbase station and a second base station, said mobile communicationsdevice having a first wireless communications link with the first basestation at the time the handoff is initiated.

FIG. 13 is a flowchart of an exemplary method, in accordance with thepresent invention, of operating a mobile node to implement a handoffbetween a first link with a first base station sector and using a firstcarrier and a second link with a second base station sector, said secondlink using a second carrier, at least the first sector being differentfrom the second sector or the first carrier being different from thesecond carrier.

FIG. 14 is a flowchart of an exemplary method of implementing handoffsbetween base station sectors in accordance with the present invention.

FIG. 15 is a drawing including exemplary uplink dedicated accesssegments and exemplary uplink contention based access segments inaccordance with the present invention.

FIG. 16 is a drawing illustrating exemplary cells in an exemplarymulti-sector multi-carrier system in which multiple carriers are used inthe same sector with different power levels.

FIG. 17 illustrates the use of a sector which supports multiple carrierswhere beacon signals are transmitted into the frequency band of each ofthe carriers by each of the sector transmitters.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to methods and apparatus forimplementing handoffs involving a change in carrier frequencies. Thehandoffs may be between different cells, e.g., intercell handoffs,between sectors within a cell, e.g., intra-cell inter-sector handoffs orhandoffs between different carriers within a sector, e.g., intra-sectorinter-carrier handoffs. Inter-cell handoffs and intra-cell inter-sectorhandoffs often involve change of carriers.

The handoffs implemented in accordance with the present inventiongenerally involve terminating a first communications link beforecompleting the handoff and successful establishment of a secondcommunications link, e.g., using a different carrier frequency. Whilediscussed in the context of handoffs involving a change in carrierfrequencies, some aspects of the present invention can be used toimplement handoffs where the new and the old carrier frequencies usedare the same but the point of network attachment changes. For example,in the case of cells with timing synchronized sectors which use the samecarrier frequency in multiple sectors, a handoff from one sector of thecell to another sector can be implemented without the need to performtiming synchronization in the new sector before transmitting user datasince the timing synchronization remains valid even though the mobilenode changes the sector in the cell through which it attaches to thenetwork via a wireless connection.

In the exemplary system each cell includes a base station whichtransmits different signals into each sector of the cell. Cells mayinclude one or more sectors. In many embodiments a single carrierfrequency is used in each sector of a cell. However, in someembodiments, multiple carrier frequencies are used in each sector. Insuch embodiments, intra-sector, inter-carrier handoffs are possible witha mobile device switching from using the transmitter/receiver or othersignal processing circuitry associated with one carrier frequency to thetransmitter/receiver or other signal processing circuitry associatedwith another carrier frequency.

A separate antenna and/or transmitter may be provided for each sector ofa cell. In some but not all embodiments, symbol timing and carrierfrequency are synchronized across sectors of the cell. In addition, theframing structure is also synchronized across sectors of the cell sothat the slots or superslots of the signals in one sector start at afixed time offset from where those of another sector start, and thefixed time offset can be zero in some embodiments. However, symboltiming or carrier frequency is normally not synchronized across cells.The base station, in accordance with various embodiments of theinvention, transmits multiple beacon signals, e.g., at different times,from each sector of a cell. One or more beacon signals are normallytransmitted within the frequency band or bands, e.g., in the case ofmultiple carriers in a sector, used by each sector to communicateinformation to wireless terminals within the sector. Beacon signals arenarrow band signals transmitted using relatively high power, e.g., apower level higher than the average power level used to transmit userdata. In many cases the beacon signals are several times higher than theaverage user data power level. Such beacon signals can be used to conveyinformation, e.g., a sector identifier, slope which is a cellidentifier, and/or information about the carrier frequency/frequencyband used in the sector transmitting the beacon.

In some embodiments of the present invention, the base station uses asector transmitter to periodically transmit a beacon signal at apredetermined frequency within the frequency band used by an adjacentsector or cell. As a result, multiple sectors may transmit beaconsignals into the same frequency band, e.g., at different times. In thismanner, a receiver in one sector can identify the presence and thesignal strength of neighboring sectors and obtain information about thesector without having to change to a different frequency band being usedin the neighboring sector. To make it easy to distinguish the sectorwhich is the source of a beacon signal within a particular frequencyband, each sector transmits a beacon at a different predeterminedfrequency within any given frequency band used by a sector. Carrierfrequency information can be associated with a beacon in addition tosector information. The predetermined frequency with a given frequencyband may vary according to a pre-selected sequence over time. Thesequence repeats at some point, e.g., after a fixed number ofsuperslots.

The strength of the beacon signals received from adjacent sectors and/orcells, or from the same sector but corresponding to a different carrierfrequency, may be compared to the strength of the beacon signalcorresponding to the sector and carrier frequency with which the mobilehas a communication link to determine when a handoff should beperformed. In accordance with the invention, the monitoring andevaluation of beacon signals from adjacent sectors/cells, or differentcarriers of the same sector, allows the wireless terminal to, in manycases, implement a relatively seamless hand-off while avoiding arelatively lengthy disruption or interruption in service that may occurin systems where switching to a different carrier is required todetermine the carrier frequency to be used following a handoff.

In one exemplary OFDM (Orthogonal Frequency Division Multiplexed)embodiment, a beacon signal is implemented as a relatively high poweredsignal that is transmitted on a single tone, e.g., frequency, or a fewtones. The power used to transmit the beacon signal is often more thantwice the highest power signal tone used to communicate data or pilotsignals in the sector. When a beacon signal is transmitted in theexemplary OFDM embodiment, most of the transmission power isconcentrated on one or a small number of tones, e.g., a single tonewhich comprises the beacon signal. Many or most of the tones which arenot used for the beacon signal may, and often are, left unused. Thus,when transmitting a beacon signal into the frequency band used by anadjacent sector, most or all of tones used in the frequency band of thesector transmitting the beacon signal may go unused by the sector'stransmitter.

FIG. 1 shows an exemplary 3 sector cell 100 corresponding to a basestation (BS) 102 implemented in accordance with one exemplary embodimentof the present invention. Base station 102 is a sectorized base station.The base station (BS) 102 transmits ordinary signals into sector 1 106using carrier frequency f₁. The BS 102 transmits ordinary signals intosector 2 108 using carrier frequency f₂, and ordinary signals intosector 3 110 using carrier frequency f₃. A wireless terminal (WT) 104,implemented in accordance with the present invention, is shown on theboundary area between sector 1 106 and sector 2 108. Handoffs of WT 104may be performed between different base station sectors of the same cellin accordance with the methods of the present invention.

FIG. 2 shows three exemplary cells (Cell 1 202, Cell 2 204, Cell 3 206)in an exemplary wireless communications system 200 in accordance withthe present invention. Each cell includes a base station and 3 sectors,each of the three sectors uses a different carrier frequency (f₁, f₂,f₃) and corresponding frequency band for ordinary communications withwireless terminals within the particular sector. The same three carrierfrequencies f₁, f₂, f₃, and bandwidth associated with each carrier isreused in each of the cells. Cell 1 202 includes base station 1 (BS1)208 and 3 sectors (sector 1 214, sector 2 216, sector 3 218) usingcarrier frequencies (f₁, f₂, f₃), respectively. Cell 2 204 includes basestation 2 (BS2) 210 and 3 sectors (sector 1 220, sector 2 222, sector 3224) using carrier frequencies (f₁, f₂, f₃), respectively. Cell 3 206includes base station 3 (BS3) 212 and 3 sectors (sector 1 226, sector 2228, sector 3 230) using carrier frequencies (f₁, f₂, f₃), respectively.FIG. 2 also includes an exemplary wireless terminal (WT) 232,implemented in accordance with the present invention. The WT is situatedon the boundary between sector 1 214 of cell 1 202 and sector 2 222 ofcell 2 204. Handoffs of WT 232 may be performed between different basestation sectors of different cells or between different base stationsectors of the same cell in accordance with the methods of the presentinvention.

The total frequency band of the FIG. 2 example is subdivided into 3frequency bands (slots) situated contiguously and is identical in eachsector. In general, the total frequency band need not be identical ineach sector, and the frequency bands (slots) may be disjoint and neednot be identical in each sector. In some embodiments, the BSs 208, 210,212 transmit beacon signals. A beacon signal, in various embodiments, isimplemented as one or more narrowband high power broadcast signals. Insome embodiments, the beacon signal transmission in each sector, whenscheduled, may alternate between the 3 frequency ranges (bands) overtime. In other embodiments, the base station shall in each sector beable to transmit beacon signals in more than one of the carrierfrequency bandwidth ranges (bands) with beacons being transmitted inmultiple frequency bands from sector transmitter simultaneously.

FIG. 3 shows three graphs 302, 304, 306 indicating exemplary basestation sector transmission signaling vs frequency. The exemplarysignaling may be transmitted in a cell such as the exemplary cell 100shown in FIG. 1 or in any of the exemplary cells (202, 204, 206) shownin FIG. 2.

The top graph 302 of FIG. 3, shows signaling from base station sector 1.The graph 302 is a composite of signals which may be transmitted atdifferent times, e.g., during different symbol transmission periods.First frequency band 310 which is centered around carrier frequency f₁is used for transmitting signals and information to wireless terminalsin sector 1 as indicated by the label ordinary signaling 319.Periodically, e.g., when not transmitting data, e.g., ordinary signals,the transmitter in sector 1 transmits a beacon signal S1F1 (Sector 1carrier Frequency 1) 320 within the first frequency band. This frequencymay be a fixed offset from the first carrier frequency and can be usedby the wireless terminals to identify and synchronize with the carrierfrequency being used in the first sector. To provide information to WTsin neighboring sectors where carrier f₂ is used, periodically, the firstsector transmitter transmits a beacon signal S1F2 322 at a predeterminedfrequency within the second frequency band 312 corresponding to thesecond carrier frequency f₂. This signal can be detected by WTs in theadjacent sector without those terminals having to adjust their receiverfrequency to another band, e.g., the first frequency band 310 used insector 1. In addition, to provide information to WTs in neighboringsectors where carrier f₃ is used, periodically, the first sectortransmitter transmits a beacon signal S1F3 324 at a predeterminedfrequency within the third frequency band 314 corresponding to the thirdcarrier frequency f₃. This signal can be detected by WTs in adjacentsectors where the third frequency band is used without those terminalshaving to adjust their receiver frequency to another band, e.g., thefirst frequency band 310 used in sector 1.

The middle graph 304 of FIG. 3, shows signaling from base station sector2. The graph 304 is a composite of signals which may be transmitted atdifferent times, e.g., during different symbol transmission periods.Second frequency band 312 which is centered around carrier frequency f₂is used for transmitting signals and information to wireless terminalsin sector 2 as indicated by the label ordinary signaling 331.Periodically, e.g., when not transmitting data, e.g., ordinary signals,the transmitter in sector 2 transmits a beacon signal S2F2 (Sector 2carrier Frequency 2) 332 within the second frequency band 312. Thisfrequency may be a fixed offset from the second carrier frequency andcan be used by the wireless terminals in sector 2 to identify andsynchronize with the carrier frequency being used in the second sector.To provide information to WTs in neighboring sectors where carrier f₁ isused, periodically, the second sector transmitter transmits a beaconsignal S2F1 330 at a predetermined frequency within the first frequencyband 310 corresponding to the first carrier frequency f₁. This signalcan be detected by WTs in the adjacent sector which uses the firstcarrier frequency without those terminals having to adjust theirreceiver frequency to another band, e.g., the second frequency band 312used in sector 2. In addition, to provide information to WTs inneighboring sectors where carrier f₃ is used, periodically, the secondsector transmitter transmits a beacon signal S2F3 334 at a predeterminedfrequency within the third frequency band 314 corresponding to the thirdcarrier frequency f₃. This signal can be detected by WTs in adjacentsectors where the third frequency band is used without those terminalshaving to adjust their receiver frequency to another band, e.g., thesecond frequency band 312 used in sector 2.

The bottom graph 306 of FIG. 3, shows signaling from base station sector3. The graph 306 is a composite of signals which may be transmitted atdifferent times, e.g., during different symbol transmission periods.Third frequency band 314 which is centered around carrier frequency f₃is used for transmitting signals and information to wireless terminalsin sector 3 as indicated by the label ordinary signaling 343.Periodically, e.g., when not transmitting data, e.g., ordinary signals,the transmitter in sector 3 transmits a beacon signal S3F3 (Sector 3carrier Frequency 3) 344 within the third frequency band. The frequencyof this beacon signal may be a fixed offset from the third carrierfrequency and can be used by the wireless terminals in sector 3 toidentify and synchronize with the carrier frequency being used in thethird sector. To provide information to WTs in neighboring sectors wherecarrier f₁ is used, periodically, the third sector transmitter transmitsa beacon signal S3F1 340 at a predetermined frequency within the firstfrequency band 310 corresponding to the first carrier frequency f₁. Thissignal can be detected by WTs in the adjacent sector which uses thefirst carrier frequency without those terminals having to adjust theirreceiver frequency to another band, e.g., the third frequency band 314used in sector 3. In addition, to provide information to WTs inneighboring sectors where carrier f₂ is used, periodically, the thirdsector transmitter transmits a beacon signal S3F2 342 at a predeterminedfrequency within the second frequency band 312 corresponding to thesecond carrier frequency f₂. This signal can be detected by WTs inadjacent sectors where the second frequency band is used without thoseterminals having to adjust their receiver frequency to another band,e.g., the third frequency band 314 used in sector 3.

Each beacon signal can uniquely identify the carrier associated with thesector from which the beacon signal originated and can, in variousembodiments, also provide additional information. In FIG. 3, the nineexemplary beacon signals shown are at different frequencies. Thus, it ispossible to match a frequency of a beacon signal to a set of knownbeacon frequencies to determine which sector transmitter was the sourceof a particular detected beacon signal.

In accordance with the invention, a wireless terminal, e.g., mobilenode, may receive the beacon signals from its own and different, e.g.,adjacent, sector base station transmitters. The beacon signals arereceived within the same frequency band that the wireless terminal usesfor ordinary signaling, e.g., data and/or control signaling. Beaconsignal strength (e.g., power) measurements are made in addition tofrequency measurements. Comparisons of the strength of differentreceived beacon signals from different base station sector transmittersare used by the WT to decide when to make a handoff to a different basestation sector. The beacon signal comparison also indicates to thewireless terminal which carrier frequency that the wireless terminalshould use for ordinary signaling following the hand-off. In someembodiments, this carrier frequency is determined to be the carrierfrequency used for ordinary signaling by the base station sectortransmitter which transmitted the stronger of the received beaconsignals.

Consider for example, the wireless terminal 104 shown in FIG. 1, whichis operating in sector 1, and is therefore using carrier frequency f₁and its associated bandwidth 310 for ordinary signaling, e.g., receivingand sending information to the base station. However, it is alsomonitoring for beacon signals within the frequency band 310corresponding to carrier frequency f₁. Refer to the left portion of FIG.3, showing the signaling transmitted by the BS in each of the threesectors in the first frequency band 310 corresponding to carrier f₁. Thewireless terminal 104 compares the received strength of the beaconsignal 320 from sector 1, with the received strength of adjacent sectorbeacon signals 330 and 340 which are also detected within the firstfrequency band 310. As the wireless terminal nears the boundaryseparating sector 1 and sector 2, the reception strength of beaconsignal S2F1 330 within the first frequency band transmitted by the BSsector 2, increases in strength relative to the received signal strengthfrom the sector 1 beacon signal S1F1 320. At some point, based upon thiscomparison of received beacon signal strengths and criteria within thewireless terminal, the wireless terminal may initiate a hand-off tocarrier frequency f₂, the frequency used for ordinary signaling insector 2. The wireless terminal knows to switch to carrier frequency f₂and not carrier frequency f₃ e.g., based upon the position in thefrequency domain of the stronger received beacon signal.

Signaling from each sector of the same cell may be timing synchronizedwith respect to one another. Therefore, in intra-cell inter-sectorand/or intra-cell inter-sector handoff operations, some operationsassociated with timing synchronization which are normally performed uponentering a cell or sector before user data can be transmitted need notbe performed, in accordance with the invention before user data such asvoice or text can be transmitted to the receiver corresponding to thenew sector or carrier frequency.

The same or a similar method of the invention, described with respect tohandoffs at sector boundaries, is also used with respect to handoffs atcell boundaries as in the case of the wireless terminal 232 shown inFIG. 2 situated on a cell boundary. In such a case, the handoff is fromthe sector of one cell to the sector of another cell. In regard tocells, the location of the beacon may also be used to convey cellinformation, e.g., a slope value used as a cell identifier in someembodiments. Different cells, sectors, and carriers within a sector mayuse different predetermined frequencies for beacon signals.Predetermined periodic changes in beacon signal frequency over time maybe used to communicate slope information in some embodiments. In oneembodiment, the changes in the beacon signal are changes in the beaconlocation via a hopping pattern on the tones which may indicate a slopecorresponding to a cell.

FIG. 4 shows an example where two different adjacent cells have a slightvariation in beacon frequency location designations in the same sector,exemplary sector 1, to provide beacon signal identification to a sectorand cell level. For example, drawing 402 may correspond to signalstransmitted from BS1 208 sector 1 214 of cell 1 202 transmitter of FIG.2, while drawing 404 may correspond to signals transmitted from BS2 210sector 1 220 of cell 2 204 of FIG. 2. Drawing 402 includes a bandwidthassociated with carrier frequency f₁ 406, a bandwidth associated withcarrier frequency f₂ 408, and a bandwidth associated with carrierfrequency f₃ 410. Within bandwidth for carrier f₁ 406, the BS 1 sector 1transmitter transmits a beacon signal 412 and ordinary signaling 414,e.g., user data and control signals. Within bandwidth for carrier f₂408, the BS 1 sector 1 transmitter transmits a beacon signal 416. Withinbandwidth for carrier f₃ 410, the BS 1 sector 1 transmitter transmits abeacon signal 418. The various signals 412, 414, 416, and 418 may betransmitted at different times, e.g., with the ordinary signaling 414being transmitted most of the time, and a beacon signal, from the set ofbeacon signals including 412, 416, 418, being transmitted occasionallyin a predetermined sequence on a periodic basis in place of the ordinarysignaling 414. Drawing 404 includes a bandwidth associated with carrierfrequency f₁ 406, a bandwidth associated with carrier frequency f₂ 408,and a bandwidth associated with carrier frequency f₃ 410. Withinbandwidth for carrier f₁ 406, the BS 2 sector 1 transmitter transmits abeacon signal 420 and ordinary signaling 422, e.g., user data andcontrol signals. Within bandwidth for carrier f₂ 408, the BS 2 sector 1transmitter transmits a beacon signal 424. Within bandwidth for carrierf₃ 410, the BS 2 sector 1 transmitter transmits a beacon signal 426. Thevarious signals 420, 422, 424, and 426 may be transmitted at differenttimes, e.g., with the ordinary signaling 422 being transmitted most ofthe time, and a beacon signal, from the set of beacon signals including420, 424, 426, being transmitted occasionally in a predeterminedsequence on a periodic basis in place of the ordinary signaling 422.Beacon signals 412 and 420 within the same band 406 are at differentfrequency locations allowing a wireless terminal receiving the beaconsignal to distinguish between the two cells. Beacon signals 416 and 424within the same band 408 are at different frequency locations allowing awireless terminal receiving the beacon signal to distinguish between thetwo cells. Beacon signals 418 and 426 within the same band 410 are atdifferent frequency locations allowing a wireless terminal receiving thebeacon signal to distinguish between the two cells.

Cells need not be, and generally are not, timing synchronized withrespect to one another. Therefore, in inter-cell handoff operations, thewireless terminal is normally required to perform timing synchronizationoperations before transmitting user data so that symbols, e.g., symbolscarrying user data, that are transmitted by the mobile are asynchronized manner at the BS with symbols transmitted by other mobiles.Beacon signals or other broadcast signals may be used in achievingcoarse timing synchronization and minimizing break time during handoffoperations in accordance with the present invention.

FIG. 5 shows an exemplary communications system 500 implemented inaccordance with the present invention which utilizes the methods of thepresent invention. The exemplary system includes a plurality of cells(cell 1 502, cell M 504). Each cell represents the wireless coveragearea for an access node, e.g., a base station. Cell 1 502 corresponds tobase station 1 506 and cell M 504 corresponds to base station M 508.Each cell is subdivided into a plurality of sectors. The exemplarysystem shows a 3 sector embodiment; however, in accordance with theinvention, cells with less or more than 3 sectors are also possible. Theexemplary system uses a different carrier frequency in each of thesectors of a cell. In other embodiments, frequencies may be reused bysectors within a cell, e.g., reused by those sectors that are notadjacent. Alternatively, in some embodiments multiple carriers are usedin each section with different power levels being used for a particularcarrier in adjacent sectors which uses the same carrier frequencies. Inthe illustrated example of FIG. 5 sector 1 uses carrier frequency f₁;sector 2 uses carrier frequency f₂; sector 3 uses carrier frequency f₃.The same carrier frequencies are used in the same sectors e.g., sectors1, 2, and 3, of other cells of the exemplary system.

In some embodiments, the carrier frequencies used in different cells ofthe system may vary slightly. In still other embodiments, the carrierfrequencies used in different cells may be substantially different. Cell1 502 includes sector 1 510, sector 2 512, and sector 3 514. Cell M 504includes sector 1 516, sector 2 518, and sector 3 520. An exemplaryboundary region 522 is shown where cell 1 sector 1 510 overlaps withcell M sector 2 518, in which inter-cell handoff operations are likelyto occur, in accordance with the methods of the present invention.Handoff operations may also occur in boundary areas between thedifferent sectors of the same cell, in accordance with the methods ofthe present invention.

The exemplary system of FIG. 5 also includes a plurality of end nodesEN1, EN N, e.g., wireless terminals such as mobile nodes, in each of thesectors of each cell. The wireless terminals are coupled to the basestations via wireless links. If the end nodes are mobile devices, theymay move throughout sectors and cells of the system. The end nodes mayinitiate and perform handoff operations from one base station sectorattachment point to another base station sector attachment point, inaccordance with the methods of the present invention. Mobile devices aresometimes referred to herein as mobile communications devices or mobilenodes. Cell 1 502 sector 1 510 includes a plurality of ENs (EN1 524, ENN 526); cell 1 502 sector 2 512 includes a plurality of ENs (EN1 528, ENN 530); cell 1 502 sector 3 514 includes a plurality of ENs (EN1 532, ENN 534). Cell M 504 sector 1 516 includes a plurality of ENs (EN1 536, ENN 538); cell M 504 sector 2 518 includes a plurality of ENs (EN1 540, ENN 542); cell 1 504 sector 3 520 includes a plurality of ENs (EN1 544, ENN 546).

The access nodes (base stations) (506, 508) are coupled to a networknode 548, e.g., a router, via network links (550, 552), respectively.Network node 548 is coupled to other network nodes and the Internet vianetwork link 554. Network links (550, 552, 554) may be, e.g., fiberoptic cables.

Sector boundary regions are identified as dividing lines within eachcell separating the three sectors (510, 512, 514) or (516, 518, 520),and exemplary cell boundary region 522 is shown as an overlapping areabetween cell 1 and cell M. As wireless terminals travel throughout thesystem and approach and/or traverse sector and/or cell boundarieshandoff operations involving a change in carrier frequency may beperformed in accordance with the invention.

In accordance with the invention the base stations (506, 508) areperiodically transmitting beacon signals in each of three frequencybands (associated with the three carrier frequencies f₁, f₂, f₃) intoeach sector of each cell. In accordance with the invention, the endnodes (524, 526, 528, 530, 532, 534, 536, 538, 540, 542, 544, 546) aremonitoring the beacon signals in the frequency band of currentoperation, in order to make decisions regarding inter-sector,intra-sector (if multiple carriers are used in a sector) and/orinter-cell handoffs.

FIG. 6 illustrates an exemplary access node (base station) 600implemented in accordance with the present invention. The base station600 of FIG. 6 may be a more detailed representation of any of the basestations of the system of FIG. 1, 2 or 5. The base station 600 includesa processor 602, e.g., CPU, a plurality of receivers, e.g., one for eachsector of the base station 600 (sector 1 receiver 604, sector 2 receiver606, . . . sector N receiver 608), a plurality of transmitters, e.g.,one for each sector of the base station (sector 1 transmitter 610,sector 2 transmitter 612, . . . sector N transmitter 614), an I/Ointerface 616, a clock module 618, a memory 620, and, in someembodiments, a plurality of beacon transmitters, e.g., one for eachsector of the base station (beacon sector 1 transmitter 622, beaconsector 2 transmitter 624, beacon N transmitter 626), coupled togethervia a bus 628 over which the various elements can interchange data andinformation. Different transmitter circuitry can, and often is, includedfor each carrier frequency used in a sector in the case of sectors whichsupport the use of multiple carrier frequencies. Each base stationsector receiver (604, 606, 608) is coupled to a sector antenna (sector 1receive antenna 630, sector 2 receive antenna 632, sector N receiveantenna 634), respectively, and can receive signals, e.g. uplink signalsincluding requests for handoffs, timing control signals, power controlsignals, and user data, from wireless terminals in the sector covered.Different receiver circuitry may, and often is, included for eachcarrier frequency used in a sector in the case where multiple carrierfrequencies are used in a sector. Each receiver (604, 606, 608) includesa decoder (636, 638, 640), respectively, which decodes received uplinkencoded signals to extract the information being communicated. Eachsector transmitter (610, 612, 614) is coupled to a sector antenna(sector 1 transmit antenna 642, sector 2 transmit antenna 644, sector Ntransmit antenna 646), respectively, and can transmit signals, includingdownlink broadcast signals such as, e.g., beacon signals, and userspecific downlink signals such as signals including informationidentifying dedicated resources for use in handoff operations, inaccordance with the invention, into the sector covered. Each sectortransmitter (610, 612, 614) includes an encoder (648, 650, 652),respectively, for encoding downlink information prior to transmission.In some embodiments the base station 600 includes and uses separatereceivers, transmitters, and/or antennas for each of the sectors, andoptionally, carrier frequencies in a sector, of the cell. In someembodiments, a base station uses: a single receiver with sectorizedfunctionality to receive signals from each of the sectors covered by thebase station, a single transmitter with sectorized functionality totransmit into each of the sectors covered by the base station, and/orsectorized antennas, e.g., an antenna with different elementscorresponding to different sectors. In some embodiments, sector beacontransmitters (622, 624, 626) are included and are coupled to transmitantennas (642, 644, 646), respectively; the sector beacon transmitters(622, 624, 626) are used to transmit some or all of the beaconsignaling, allowing simultaneous transmission of multiple beaconsignals, and limiting disruptions in normal ordinary signalingtransmissions by, in some embodiments, off loading some or all of thebeacon transmissions functions.

The base station I/O interface 616 couples the base station 600 to othernetwork nodes, e.g., other access nodes (base station), routers, AAAservers, home agent nodes, and the Internet. Handoff signaling iscommunicated through I/O interface 616 between base stations prior tothe termination of the current wireless link and the establishment of anew wireless link, in accordance with some embodiments of the presentinvention.

Clock module 618 is used for maintaining timing synchronization betweenthe various sectors covered by the base station. Synchronization betweenthe different sectors of the same cell allows for intra-cellinter-sector and intra-cell intra-sector inter-carrier handoffoperations to be performed in a more efficient manner, e.g., withreduced or eliminated wireless terminal timing synchronization steps, ascompared to inter-cell handoff operations in which the WT needs toperform timing synchronization steps with the new attachment pointbefore communicating power control information and/or user data.

Memory 620 includes routines 654 and data/information 656. The processor602 executes routines 654 and uses the data/information 656 in thememory 620 to control the operation of the base station 600 includingthe normal functions of scheduling, base station power control, basestation timing control, communication, signaling, and including the newfeatures of the invention including the beacon signaling and handoffoperations.

The data/information 656 in memory 620 includes a plurality of sets ofdata/information, e.g., one for each sector covered by the base station(sector 1 data/information set 658, sector N data/information set 660).Sector 1 data/information set 658 includes data 661, basestation-to-base station information 662, sector information 664, beaconinformation 666, and wireless terminal (WT) data/information 668. Data661 includes user data to be transmitted to and received from wirelessterminals. Base station-to-base station information 662 includesinformation communicated between BSs pertaining to handoff signaling andstored security information, e.g., security keys used to establish asecure link between base stations prior to conveying WT handoffinformation between base stations. Sector information 664 includescarrier information 670, e.g., carrier frequencies and bandwidthsassociated with the sector. Sector information 664 also includesresource information 672, e.g., information identifying dedicatedresources which can be allocated to WTs for use in handoff operations,e.g., base station assigned WT identifiers, uplink dedicated segmentssuch as timing control channel segments, power control channel segments,and traffic channel segments.

The beacon information 666 includes tone information 674, e.g.,information associating beacon signals in each sector with specificfrequencies, timing information 676, e.g., information identifyingbeacon signal transmission timing and information identifying timingrelationships between beacon signals and dedicated uplink resourceswhich may be assigned for use in handoff operations, and tone hoppinginformation 678, e.g., information used to generate hopping sequencesused for the beacon signals, e.g., to convey cell identificationinformation, e.g., slope.

WT data/info 668 includes a plurality of WT data/information sets foreach WT: WT 1 data/info 680, WT N data/info 682. WT 1 data/info 680includes user data 684 in route from/to WT 1, a terminal ID 686associating the WT to the base station, and sector ID information 688including information identifying the sector in which WT 1 is currentlylocated and associating WT 1 to a specific carrier frequency used forordinary signaling. Sector ID information 688 also includes informationidentifying a sector to which WT1 has requested as the new attachmentpoint in a handoff request. WT 1 data/info 680 also includes dedicatedresource information 690, e.g., information from the set of sectordedicated resource information 672, which has been allocated to WT 1 foruse in handoff operations. In different types of handoff operationsdifferent resources may be dedicated to WT 1 and included in dedicatedresources information 690. For example, an inter-cell handoff intosector 1 of BS 600 may include the allocation of a dedicated deviceidentifier to be used in the specific sector where communicating on aparticular carrier, a dedicated uplink timing channel segment and/or adedicated uplink power control channel segment to WT1, while anintra-cell inter-sector or an intra-cell intra-sector inter-carrierhandoff into or within sector 1 of BS 600 may omit the allocation of anuplink timing control channel segment to WT1 and include the allocationof an uplink power control channel segment to WT1. Handoff messages 692includes handoff messages pertaining to WT1, e.g., handoff requestmessages received directly or indirectly from WT1 requesting initiationof a different attachment point, dedicated resource allocation messagesbeing sent to WT 1 identifying resources, e.g., identifiers and/oruplink segments, that may be used establish a new wirelesscommunications link with a new attachment point, and base station tobase station secure communications link establishment messages. Modeinformation 694 includes information identifying the state of operationof WT 1, e.g., ON, Hold, Access, etc, and information identifyingwhether a wireless link has been established between WT1 and the basestation 600 sector 1, is being established, or is in a process oftermination. Mode information 694 also includes information identifyingthat a new wireless link that being established between WT1 and otherbase station and/or other sector attachment points.

Routines 654 includes a plurality of sets of routines, e.g., one foreach sector covered by the base station (sector 1 routines 651, . . . ,sector N routines 653). Routines 651 include communications routines655, and base station control routines 657. The communications routines655 implement the various communications protocols used by the basestation. The base station control routines 657, using data/information658, control base station sector 1 operation including the receiver 604,transmitter 610, optional beacon transmitter 622, I/O interface 616,scheduling, ordinary control and data signaling, beacon signaling, andhandoff operation, in accordance with the present invention. Basestation control routines 657 includes a scheduler module 659, signalingroutines 661, handoff routine 663, WT timing control module 665, and WTpower control module 667. Scheduler module 659, e.g., a scheduler,schedules air link resources, e.g. bandwidth over time in the form ofsegments, to wireless terminals for uplink and downlink communications.

Signaling routines 661 control one or more of: the receiver, thedecoder, the transmitter, the encoder, ordinary signal generation,beacon signal generation, data and control tone hopping, signaltransmission, signal reception, and handoff signaling. Signalingroutines 661 include beacon module 669 and handoff signaling module 671.The beacon module 669 uses the beacon information, e.g., sector 1 beaconinfo 666, to control the generation and transmission of beacon signalsin accordance with the invention. In accordance with the invention,beacon signals may be transmitted in each sector in each of the carrierfrequency bands used in the sector. In some embodiments, the beaconsignals are transmitted through the sector transmitters (610, 612, 614).In other embodiments, some or all of the beacon signals may betransmitted by the beacon transmitters (622, 624, 626). Handoffsignaling module 671 controls the handoff signaling, e.g., handoffmessages 692, being transmitted from and received by base station 600sector 1.

Handoff routines 663 include a request processing module 673, a securebase station-base station link establishment module 675, a dedicatedresource allocation module 677, a registration module 679, and awireless link establishment/termination module 681. Request processingmodule 673 receives and processes requests by a WT to establish a newwireless communications link with a base station sector attachmentpoint. Base station-base station link establishment module 675 uses thedata/info 656 including BS-BS info 662 to establishes a securecommunications link between BS 600 sector 1 and another base station,the secure communications link can be used to communicate handoffinformation via I/O interface 616. Dedicated resource allocation module677 allocates dedicated resources, e.g., such as resources identified inresource info 672, to a WT which has requested a handoff to sector 1 ofBS 600. Module 677 may generate information such as dedicated resourceinfo 690 and form such information into handoff messages 692 specifyingidentifiers, uplink timing control channel segments, uplink powercontrol channel segments, and/or uplink traffic channel segments, whichmay be communicated via the handoff signaling module 671 either directlyor indirectly to the WT, e.g., depending upon whether an inter-cell orintra-cell handoff operation is involved. Registration module 679 maycontrol the performance of registration operations when a WT requeststhe initiation and establishment of a new wireless link with a basestation 600 sector 1 attachment point. Different registrationoperational sequences may be used depending upon whether the handoff isinter-cell or intra-cell, e.g., with respect to whether or not timingsynchronization steps are performed. Wireless linkestablishment/termination module 681 controls operations in theestablishment and termination of wireless link to BS 600 sector 1. Forexample, in the case of the establishment of a new wireless link, module681 recognizes that a new link can be established at the time of theearliest allocated dedicated uplink segment that BS 600 sector 1 hasallocated to the WT requesting a handoff, and therefore looks for uplinksignaling from the WT at the appropriate time. In the case of thetermination of a wireless link between BS 600 sector 1 and a WT, e.g.,the termination may be based on the BS not receiving any signaling fromthe WT in a predetermined time, and module 681 performs the timeoutmeasurement and relinquishes resources, e.g., an identifier andassociated dedicated segments, following a timeout determination.Alternative termination methods are possible, e.g., the BS sector 1 canmonitor the handoff signaling corresponding to the new attachment point,e.g., handoff messages traversing I/O interface 616, and determine whenthe new wireless link is to be established and terminate based upon thatdetermined time. Alternately, the WT may communicate a terminationmessage to BS 600 sector 1.

WT timing control module 665 performs operations to control the timingof the WTs, e.g., synchronizing the WT with respect to the BS 600 sector1 so that signals may be processed and decoded. Module 665 processesreceived timing control information received on dedicated uplink timingcontrol segments allocated by BS 600 sector 1 to a WT seeking toestablish a new wireless link. In addition timing control module 665generates timing correction signals which are sent via the BS sectortransmitter over established wireless links which the WT uses to maketransmission timing adjustments.

WT power control module 667 performs operations to control the power ofWTs, e.g., uplink transmission power of a WT. WT power control module667 processes received power control information received on dedicateduplink power control segments allocated by BS 600 sector 1 to a WTseeking to establish a new wireless link.

FIG. 7 illustrates an exemplary wireless terminal (end node) 700 such asa mobile node, implemented in accordance with the present invention. Thewireless terminal 700 of FIG. 7 may be a more detailed representation ofany of the end nodes of the systems of FIG. 1, 2, or 5. The wirelessterminal 700 includes a receiver 702, a transmitter 704, a processor706, e.g., CPU, user input/output (I/O) devices 708, and memory 710coupled together via a bus 712 over which the various elements caninterchange data and information. The receiver 702 including a decoder714 is coupled to an antenna 716 over which the wireless terminal 700may receive downlink signaling including beacon signaling and handoffmessages including information identifying dedicated resourcestransmitted from base stations 600 in accordance with the invention. Thedecoder 714 in the receiver 702 may decode ordinary signaling intendedfor WT 700 and use error correction coding processes to attempt torecover information overwritten or interfered with by other signalsincluding beacon signaling. The transmitter 704 including an encoder 718is coupled to an antenna 720 and may transmit signals including encodedinformation to the base station 600 including requests to initiate ahandoff of WT 700 to another base station sector attachment point,timing synchronization information over dedicated uplink timing channelsegments, power synchronization information over dedicated uplink powercontrol channel segments, and user data over dedicated uplink trafficchannel segments. Different types of handoffs are possible, inaccordance with the present invention, said handoffs including one ormore of the following characteristics: inter-cell, inter-sector, and/orinter-carrier.

User I/O devices 708, e.g., speaker, microphone, keyboard, keypad,display, mouse, video camera, etc, provide the user of WT 700 theability to input user data/information intended for peer nodes and toaccess user data/information received from peer nodes. The wirelessterminal's memory 710 includes routines 722 and data/information 724.The processor 706 executes the routines 722 and uses thedata/information 724 in memory 710 to control the operation of thewireless terminal 700 including implementing the beacon functions andhandoff operations of the present invention.

Wireless terminal data/information 724 includes user data 726 such asvoice, text, or other types of data, and/or files intended, e.g., to besent to/or received from a peer node in a communications session withthe wireless terminal 700. Data/information 724 also includes currentbase station sector user information 728, new base station sector userinformation 730, and system information 732.

Current BS sector user information 728 includes terminal ID information734, base station ID information 736, sector ID information 738, modeinformation 740, identified beacon information 742, received timingcorrection signal information 744, and determined time to terminatewireless link 746. The terminal ID information 734 may be an identifieror identifiers, assigned to the WT 700 by the base station sector towhich the WT 700 is currently coupled via a wireless link thatidentifies the wireless terminal 700 to the base station sector. Basestation ID information 736 may be, e.g., a base station identifier,e.g., a slope value associated with the base station and used in hoppingsequences. Sector ID information 738 includes information identifyingthe sector ID of the sectorized base station's transmitter/receiverthrough which ordinary signaling is being communicated and correspondsto the sector of the cell in which the wireless terminal is located.Carrier frequency information (CF) 735 indicating the carrier frequencyto be used for the current communication link is also sometimes storedin information 728 of data/information 724 in memory 7 10. Modeinformation 740 identifies whether the wireless terminal is in anon/hold/sleep state. Identified beacon information 742 may include:information on each of the beacon signals that have been received andmeasured, e.g., cell/sector ID, signal strength level, filtered signalstrength level, and carrier frequency associated with ordinary signalingin the sector from which the beacon signal was transmitted. Identifiedbeacon information 742 may also include information identifying thecurrent attachment point sector beacon, information from comparingadjacent sector beacons to the current WT sector beacon, and informationfrom comparing measured beacon signals and/or information derived frommeasured beacon signals to handoff criteria. Received timing correctionsignal information 744 includes timing correction signals received overthe established wireless link and transmission timing adjustmentinformation used to correct the timing of signals transmitted by WT 700over the established wireless link. Determined time to terminatewireless link 746 is the time determined by WT 700 to terminate itsestablished wireless link, during handoff, e.g., based on signalingreceived over the air from the new base station sector attachment pointsuch as, e.g., beacon signaling and allocated dedicated uplink segmentsand/or through communication received over an existing link with acurrent base station.

New BS sector user information 730 includes terminal ID information 748,base station ID information 750, sector ID information 752, modeinformation 754, identified beacon information 756, dedicated resourceinformation 758, handoff messages 760, and handoff type information 762and carrier frequency information (CF) 759. The terminal ID information748 may be an identifier or identifiers, assigned to the WT by the basestation sector to which the WT 700 has requested that a handoff beinitiated to, that identifies the wireless terminal 700 to that basestation sector. Base station ID information 750 may be, e.g., a value ofslope associated with the new base station and used in hoppingsequences. Sector ID information 752 includes information identifyingthe sector ID of the new attachment point's sectorized base station'stransmitter/receiver through which ordinary signaling will becommunicated via a new wireless link. Mode information 754 identifiesthe state of operation of the WT with respect to the new BS sectorattachment point, e.g., transmitting handoff request, waiting fordedicated resource allocation, receiving and processing dedicatedresources such as assigned identifier and/or assigned dedicated uplinksegment, performing handoff operations such as transmitting timingcontrol and/or power control signaling on dedicated uplink channelsegments, handoff complete, transmitting user data, hold state, onstate, sleep state. Identified beacon information 756 includesinformation such as timing information pertaining to a received beaconfrom the new BS sector attachment point. The timing relationshipexisting between the new BS sector attachment point beacon signal anddedicated uplink segments that may be allocated as resources to WT 700,e.g., in a handoff operation, allows for WT 700 to determine the pointin time to terminate the current established wireless link and startuplink signaling to the new BS sector attachment point establishing anew wireless link, such that the interruption interval during thehandoff process can be minimized.

Dedicated resource information 758 includes information, e.g., a BSsector assigned WT identifier and/or information identifying dedicateduplink channel segments, from the new BS sector attachment point, whichhas been allocated to WT 700 for use in a handoff operation. Indifferent types of handoff operations different resources may bededicated to WT 700 and included in dedicated resources information 758.For example, in an inter-cell handoff information 758 may includeinformation identifying a dedicated uplink timing channel segment and anuplink power control channel segment to WT 700, while in an intra-cellinter-sector handoff information 758 may omit the allocation of anuplink timing control channel segment to WT 700 and include theallocation of an uplink power control channel segment to WT 700. Handoffmessages 760 includes handoff messages pertaining to WT 700, e.g., ahandoff request initiation message to be transmitted to the newrequested BS sector attachment point via the current establishedwireless link and BS sector, and then through the backhaul link. Handoffmessages 760 may also include dedicated resource allocation messagesoriginally sourced from the new base station sector attachment point,transmitted base station to base station via the backhaul link, andreceived from the current base station sector attachment point via thecurrent wireless link, such messages identifying resources, e.g.,identifiers and/or uplink segments, that may be used establish a newwireless communications link with a new base station sector attachmentpoint. Handoff type information 762 includes information identifying thetype of handoff requested, e.g., an inter-cell handoff operation, anintra-cell inter-sector handoff operation, or an intra-cellinter-carrier handoff operation. Inter-cell and inter-sector handoffs,in some-embodiments, are also distinguished by whether or not thehandoff operation is an inter-carrier handoff operation.

System information 732 includes beacon ID information 764, handoffcriteria 766, cell/sector ID information 768, beacon/dedicated segmenttiming information 770, and handoff type/operation information 772.System information 732 includes structural information of the wirelesscommunications system, e.g., base station frequency usage, timingstructures and repetition intervals. The beacon ID information 764includes information, e.g., look-up tables, equations, etc, associatingspecific sector/cell beacons in the communication system to specificfrequencies at specific times, allowing the WT 700 to identify thereceived beacon signal or signals. Handoff criteria 766 may includethreshold limits used by the wireless terminal 700 to trigger a hand-offrequest to an adjacent sector/cell, e.g., a minimum threshold on thestrength level of the beacon signal from the adjacent sector and/or athreshold level on the comparative strength of the adjacent sectorreceived beacon signal with respect to the WT's own current sectorreceived beacon signal strength. Cell/sector ID information 768 mayinclude information used to construct hopping sequences used in theprocessing, transmission, and reception of data, information, controlsignals, and beacon signals. Cell/sector ID information 768 alsoincludes carrier information 774. Carrier information 774 includesinformation associating each sector/cell of the base stations in thecommunications system with a specific carrier frequency, bandwidth, andset of tones. In some embodiments, a base station sector uses differentnon-overlapping sets of tones for uplink and downlink signaling.Beacon/dedicated segment timing information 770 includes informationdefining timing relationships between the beacon signals transmitted bythe BS sectors and the dedicated uplink segments which may be allocatedby the BS sector to WT 700 for use in a handoff. Handoff type/operationinformation 772 includes information identifying steps or sequences ofsteps that are performed as a function of the type of handoff. Forexample, an inter-cell handoff may include a timing synchronization stepthat is omitted in an intra-cell handoff.

Routines 722 include communications routine 776 and wireless terminalcontrol routines 778. Wireless terminal control routines 778 includessignaling routines 780 including beacon routines 782, handoff routines784, user data signaling module 786 and ongoing wireless terminal timingcontrol module 788. Wireless terminal communications routine 776implements the various communication protocols used by the wirelessterminal.

Wireless terminal control routines 778 performs the basis controlfunctionality of the wireless terminal including power control, timingcontrol, signaling control, data processing, I/O, control of the beaconrelated functions, and control of handoff signaling and operation inaccordance with the present invention. The signaling routines 780, usingthe data/information 724 in memory 710, control the operation of thereceiver 702 and transmitter 704 to perform operations including beaconsignal reception and processing, handoff signaling and processing, anduser data signaling and processing.

The beacon routines 782 include a beacon processing and ID module 790, abeacon strength measurement module 792, a beacon comparison module 794,and a handoff decision module 796. The beacon processing and ID module790, using the system information 732 including beacon ID information764 and cell/sector ID information 768, identifies a received beaconsignal and stores the information in the user's identified beacon info742. The beacon signal strength measuring module 792 measures the signalstrength of a received beacon signal and stores the information in theuser's identified beacon information 742. The beacon comparison module794 compares identified beacon information 742 in order to obtaininformation that may be used to determine when to initiate a handoff toan adjacent sector/cell. The beacon comparison module 794 may compareindividual beacon signal strength levels to minimum threshold levels inthe handoff criteria 766. The beacon comparison module 794 may comparerelative signal strength levels between a WT's own beacon signal and anadjacent sector/cell beacon signal. The beacon comparison module 794 maycompare the relative strength level difference measurements to thresholdlevels in the handoff criteria 766. The handoff decision module 796receives output information from the beacon comparison module 794 andmakes decisions as to whether or not to initiate a handoff request andto which base station sector using which carrier frequency to initiatethe handoff request. Handoff decision module 796 may consider otherinformation such as in process user data sessions when considering thetime to initiate the request so as to minimize disruptions.

The handoff routines 784, when triggered by output from the handoffdecision module 796 generate signaling to initiate an inter-sector,inter-cell, and/or inter-carrier handoff and perform operations tocomplete the handoff. The carrier frequency and base station sectorattachment point for the new wireless link to be used following hand-offare normally identified, in various embodiments, using beacon signals asdiscussed elsewhere. Handoff routines 784 include a request module 701,a dedicated resource module 703, a registration module 705, a wirelesslink establishment/termination module 707, a wireless terminal timingcontrol module 709 and a wireless terminal power control module 711.

Request module 701 generates requests by WT 700 to initiate andestablish a new wireless communications link with a different basestation sector attachment point. Dedicated resource module 703 receivesand processes received signals including signals identifying dedicatedresources, e.g., identifiers and/or dedicated uplink segments, allocatedto WT 700 by the new BS sector attachment point for handoff operations.Module 703 may receive handoff messages 760 from which dedicatedresource information 758 may be extracted and stored. Such informationin handoff messages 760 specifies identifiers, uplink timing controlchannel segments, uplink power control channel segments, and/or uplinktraffic channel segments. Registration module 705 uses thedata/information 724 including handoff type information 762 and handofftype/operation information 772 to control the performance ofregistration operations by WT 700 requesting the initiation andestablishment of a new wireless link with a base station sectorattachment point. Different registration operational sequences may beused depending upon whether the handoff is inter-cell or intra-cell,e.g., with respect to whether or not timing synchronization steps areperformed. Registration module 705 may also include signaling to thehome agent associated with WT 700 identifying the new attachment pointat the appropriate time. Wireless link establishment/termination module707 controls operations in the establishment of the new wireless linkand termination of the old wireless link with respect to the handoff.For example, in the case of the establishment of a new wireless link,module 707 recognizes that a new link can be established at the time ofthe earliest allocated dedicated uplink segment that has been allocatedto the WT requesting a handoff by the new base station sector attachmentpoint, and therefore establishes the new link by performing uplinksignaling at the assigned time. In the case of the termination of anestablished wireless link as part of handoff operations, e.g., thetermination may be performed by WT 700 ceasing transmissions over theestablished wireless link at a appropriate time, e.g., a time just priorto the to the occurrence of the earliest dedicated uplink segment whichhas been allocated to WT by the new BS sector attachment point. Thetiming of a received beacon signal stored in information 756 and itsknown relationship to a dedicated resource identified in information 758which was allocated by the new BS sector to WT 700 can be used incombination with beacon to dedicated segment timing information 770,which indicates an offset between the dedicated resource and the beacon,to determine the termination time, e.g., so that termination will occurshortly before the time a resource dedicated to the WT for establishinga new link can be used Alternative termination methods are possible;e.g., WT 700 may communicate a termination message over the originalwireless link to the base station sector attachment point to beterminated just prior to communicating on the earliest dedicated uplinksegment to the new BS sector. In another embodiment, the new BS sectorcan communicate a termination message over the backhaul BS-to-BS link tothe original BS sector WT attachment point upon successfully receivinguplink signaling from the WT during the allocated dedicated segment.

WT timing control module 709 performs operations to control the timingof WT 700, e.g., synchronizing WT 700 with respect to the new BS sectorattachment point so that signals may be processed and decoded. Module709 generates and sends timing control information on dedicated uplinktiming control segments allocated by the new BS sector attachment pointas part of a timing synchronization operation. In response to timingsignals received from BS, the WT timing control module 709 will modifysymbol transmission timing, e.g., a clock used to control symboltransmission timing so that symbols are received at the BS fromdifferent WTs in a synchronized manner. WT power control module 711generates and sends power control signals on dedicated uplink powercontrol segments allocated by the new BS sector attachment point as partof a WT power control operation. Thus, module 711 is responsive to powercontrol signals received from the BS to adjust the WT transmission powerlevel e.g., as part of a power control operation. Modules 709 and 711,in addition to generating and sending control signals, also, in someembodiments, receive and process control signals from the new BS sectorattachment point as part or WT timing and/or power control operations,e.g., adjusting WT transmission timing and/or WT transmission power aspart of the handoff operations.

User data signaling module 786 performs operations including usingdedicated resources, e.g., dedicated uplink traffic channel segments,allocated to the WT 700 for the new wireless link by the new BS sectorattachment point, to control the transmission of user data over the newwireless link. Ongoing wireless terminal timing control module 788 isused by an established wireless communications link to maintain timingcontrol between the current BS sector attachment point and WT 700 inwhich the module receives and processes timing control signals whichhave been communicated over the established wireless link. Theprocessing of module 788 includes, e.g., operating WT 700 to make atransmission timing adjustment to adjust the timing of signals e.g.,symbols, transmitted by WT 700 over the established wireless link. Insome embodiments, intra-cell inter-sector handoff operations and/orintra-sector inter-carrier handoff operations performed by WT 700 canuse the timing synchronization performed by module 788 or perform apredetermined adjustment based on the module 788 information, e.g., afixed offset, so that dedicated resources for timing adjustment need notbe, and are not, allocated to and used by WT 700 by the new BS sectorattachment point prior to the allocation and use of at least one uplinksegment which is used to communicate non-timing control data. In such asembodiment, in the case of an intra-cell handoff, the WT can terminatean existing link, establish a new link with a new carrier or sector andtransmit power control signals and/or user data, prior to changing itstransmitter timing in response to a timing control signal transmittedover the air from the new BS sector attachment point.

Depending on the particular embodiment, a base station may not transmitbeacon signals corresponding to each of the system frequency bands intoa given sector. In some embodiments, a base station may limit the beaconsignals transmitted into a given sector to a subset corresponding to thefrequency bands used by its own sector and adjacent sectors. In someembodiments, with regard to individual sectors, a base station may limitthe beacon signals transmitted into a given sector to a subsetcorresponding to the frequency bands used in adjacent sectors.

Although shown for a communications system with bandwidth dividedbetween 3 carrier slots (frequency bands), the invention is applicableto other communications systems in which the same frequency band is notused everywhere in the system.

In some embodiments, various features or elements of the invention maybe implemented in part of a communications system and not implemented inother parts of the system. In such an embodiment, the wirelessterminals, implemented in accordance with the invention, may utilize thebeacon signaling features and method of the invention when available inmaking decisions regarding inter-sector and/or inter-cell handoff.

Various features of the handoff methods and apparatus of the inventionwill now be described with reference to FIGS. 6-11.

In the case of non-sectorized cells each cell is normally served by asingle base station. In the case of sectorized cells, each sector may beserved by a different base station or a sectorized base station may beemployed. FIG. 6 shows an exemplary sectorized base station (accessnode) 600 where each sector is serviced by a separate receiver (sector 1receiver 604, sector 2 receiver 606, . . . , sector N receiver 608), andtransmitter (sector 1 transmitter 610, sector 2 transmitter 612, . . . ,sector N transmitter 614) which are coupled to different antennas usedin each sector. Alternatively, each sector receiver may be coupled to adifferent portion, e.g., element, of a sectorized antenna, where eachportion corresponds to a sector. Similarly, each sector transmitter maybe coupled to a different portion, e.g., element, of a sectorizedantenna, where each portion corresponds to a sector. In someembodiments, e.g., where uplink and downlink signals use differentnon-overlapping sets of tones for a given sector, receivers andtransmitters for the given sector may use the same antenna or antennaportion.

Thus, in the case of a sectorized base station embodiment 600, thecell's base station 600 includes one receiver and transmitter persector, each of which includes an analog filter, along with associatedroutines, modules and data/information that operate on a per sectorbasis to handle mobile node registration and other operations in theindividual sectors. Thus, base station 600 includes multiple sets ofroutines (sector 1 routines 651, . . . sector N routines 653) andmultiple sets of data/information (sector 1 data/information set 658, .. . , sector N data/information set 660), one per sector. Intra-cellinter-sector handoffs from one sector to another may be viewed as ahandoff from a base station sector or module included thereincorresponding to a first sector, to a base station module correspondingto a second sector of the same cell.

The use of a single base station 600 in a sectorized cell, in someembodiments, facilitates timing synchronization between sectors of thecell. Common clock circuitry included in clock module 618 may be sharedbetween the base station module's which make up a multisector cell sothat symbol timing and other operations in the individual sectors of thecell are synchronized. In the case of intra-cell handoffs, when symboltiming through the different sectors of a cell is maintained, it ispossible to reduce or eliminate the need to perform an initial timingsynchronization operation when performing a handoff since timingsynchronization remains reliable. Accordingly, at least in someembodiments, the time required to implement an intra-cell handoff isreduced by avoiding timing synchronization operations which are usedwhen an unsynchronized mobile device enters the system The intra-cellhandoff may be an inter-sector handoff. Thus, intra-cell handoffs can beimplemented in less time and/or using less resources than an inter-cellhandoff.

For purposes of explaining the invention it should be appreciated thateach cell includes at least one sector and one base station. In someembodiments multi-sector cells and base stations 600 as shown in FIG. 6are used. A sector can support multiple carrier frequencies in someembodiments. Handoffs occur between sectors or between carriers in asector. In the case of multi-sector cells, intra-cell as well asinter-cell handoffs may occur. Handoffs involve transfers ofinformation, physical layer signaling including, e.g., device IDassignments for a sector and/or carrier within a sector, and othersignaling layer operations, e.g., power and/or timing control which areperformed by the module(s) of the sector(s) involved in the handoff.Data may be communicated from one sector to another via communicationslinks, e.g., non-wireless links such as fiber optic or wire links, thatexist between one or more base stations and/or between the modulescorresponding to the sectors of a single base station.

It will be assumed for purposes of discussion that adjoining cells usedifferent frequencies. However, the handoff method of the presentinvention can be used in systems with frequency reuse factors of one,e.g., with the steps relating to making filter/receiver changes toaccommodate a different frequency being omitted from the handoffprocessing in the implementations where the same frequencies are used indifferent, e.g., adjoining sectors.

FIG. 9 is a drawing of an exemplary system 900 including a first basestation (BS1) 901, a second base station (BS2) 903, a WT 902, and aMobile Internet Protocol (IP) Home Agent (HA) node 914, implemented inaccordance with the present invention. The BSs 901, 903 may be similarto or the same as exemplary BS 600, while WT 900 may be similar to orthe same as exemplary WT 700.

Using various methods, a mobile node such as the wireless terminal (WT)902 shown in FIG. 9, engaged in an existing communications session in afirst base station sector 904, via first base station (BS1) 901, mayidentify a cell and/or sector 906 of second base station (BS2) 903(and/or a sector carrier if multiple carriers are supported in a sector)to handoff to, e.g., because of better signal conditions exist betweenthe identified cell or sector 906 than with the current cell or sector904. For purposes of explaining the invention, the discussion will belimited for the time being to examples where a single carrier is used ineach sector. For discussion purposes, the base station sector 904 withwhich a mobile device, WT 902, is communicating via wireless signaling,e.g., radio signaling, using current wireless link 950 will be describedas the “current base station sector”. The mobile device, WT 902, hasnetwork connectivity through the wireless connection 950 to the currentbase station sector 904 and via links 920, 924, 922 to other basestation sectors in the same or other cells. The base station sector towhich a mobile node, WT 902, seeks to complete a handoff will bereferred to as the “new base station sector”, and is base station sector906 in this example. In the case where there is one sector per basestation, e.g., as in the case of single sector cells, the new basestation sector will be the new base station to which a handoff operationis to be completed. In the case of multi-sector cells the new basestation sector may be part of a new base station or a different basestation sector within the same cell as the current base station sector.

In accordance with various embodiments of the present invention, eachsector of a base station periodically transmits a beacon signal into thefrequency band, e.g., f₁ band, f₂ band or f₃ band, used by the currentsector and by a physically adjacent sector. Drawing 802 of FIG. 8illustrates exemplary downlink beacon signals (beacon 1 808, beacon 2810, . . . , beacon N 812) from a base station sector transmitter on thevertical axis 804 vs time on the horizontal axis 806. The transmissionof a beacon signal for a given base station sector transmitter into afrequency band occurs in the example at least once during a firstplurality of symbol times, sometimes called a beacon slot. In theexemplary embodiment, each base station sector transmitter transmits abeacon signal during a beacon slot. A sequence of beacon signals inwhich the beacon signal transmitted during one beacon slot uses adifferent tone or tones than the beacon signal transmitted in anotherbeacon slot within the sequence may be used. The sequence of beaconsignals transmitted by a sector transmitter may include different typesof beacon signals, e.g., a beacon signal associated with carrier f₁, abeacon signal associated with carrier f₂, and a beacon signal associatedwith carrier frequency f₃. Other types of beacon signals are alsopossible in accordance with the present invention, e.g., a beacon signalwhich used to convey cell and/or sector information. The sequence ofbeacon signals repeats for each ultraslot which includes N beacon slots,where N is a positive integer. In the example, each beacon slot includes8 superslots; each superslot includes a fixed positive number of OFDMsymbol times, e.g., 113 OFDM symbol times. Superslots are shown by row814, where 8 superslots are included in a beacon slot, and a beacon istransmitted at a fixed predetermined time within each eighth superslot.Beacon slots are shown by row 816, which include multiple beacon slots,and ultra slots are shown by row 818. The beacon signal within aparticular index value beacon slot of the ultra slot repeats from ultraslot to successive ultra slot. A physically adjacent sector, whichtransmits its own set of beacon signals, may be of the current (presentattachment point) cell or an immediately neighboring cell.

Drawing 820 illustrates uplink frequencies (tones) for access segmentson the vertical axis 822 vs time 806 on the horizontal axis. It shouldbe noted that there is a time offset 824 between the start of asuperslot on the downlink and the start of a corresponding interval onthe uplink. In this example, corresponding to each superslot, there is aset of twelve access segments which may be assigned by the base stationsector attachment point as a dedicated resource to a wireless terminalwhich has requested a handoff operation to the base station sectorattachment point. Exemplary sets of access segments (826, 828, 830, 832,834, 836, 838, 840, and 842) are shown in drawing 820. By usingdedicated periods of time corresponding to access slots interferencewith transmissions by WTs already registered in the cell is minimized.Access segments are segments in which WTs entering a sector arepermitted to begin transmitting, e.g., for purposes of registering inthe sector, performing initial timing control operations, and/orperforming initial power control operations in a sector.

Each set of access segments occurs during an access slot, e.g., set 826occurs during access slot 868. The set of twelve access segments 826includes access segments (844, 846, 848, 850, 852, 854, 856, 858, 860,862, 864, and 866). The access segments corresponding to a base stationsector attachment have a fixed timing relationship with respect to thebeacon signals transmitted by that base station sector transmitter. Insome embodiments, the access segments corresponding to a base stationsector attachment point have a fixed timing relationship with respect toother beacon signals transmitted by the same base station. Note thatbecause the beacon signals of different carriers transmitted by the samebase station are synchronized with fixed timing relationships, theaccess segments of one carrier have a fixed timing relationship withrespect to the beacon signals transmitted by the same base station intoother carrier bands used by the base station and not just the beaconsignal transmitted into the same frequency band to which the accesssegments correspond. This known relationship may be used by the wirelessterminal involved in the handoff operation in determining the point intime to terminate the wireless link with the currently connected basestation sector attachment point and to start to communicate over the newwireless link on the uplink using the assigned uplink access segment.Timing offset 870 shows an exemplary offset between beacon signal 1 808and the earliest access segments of set 830. Each access segmentincludes one or more symbol times and uses one or more tones. In theexemplary embodiment each of the access segments, includes the samenumber of tone-symbols, a tone-symbol being a basic unit of air linkresource representing one tone for one OFDM symbol interval time. Inother embodiments different number of access segments may be availableand different types of access segments, e.g., for different purposes mayinclude different numbers of tone-symbols. For example an access segmentfor timing control operations may have different characteristics than anaccess segment for power control operations. Each access segment is adedicated segment dedicated for mobile device access uplink signals,e.g., registration, operations, e.g., device ID assignment, timingcontrol and/or power control operations where a device entering a sectorcan perform such operations, e.g., using one or more of a plurality ofsegments (844, 846, 848, 850, 852, 854, 856, 858, 860, 862, 864, 866)dedicated for this purpose which has been assigned to the WT by the basestation sector scheduler.

In some embodiments, access segments assigned for timing controloperations shall precede segments assigned for power control operations.For example, in the case of an inter-cell handoff operation, a WT may beassigned one segment from the set of segments (844, 846, 848, 850, 852,854) to be used to transmit timing control signal(s) and one segmentfrom the set of segment (856, 858, 860, 862, 864, 866) to be used totransmit power control signal(s). The assignment of these dedicatedresources having been conveyed to the wireless terminal via the originalwireless link, e.g., via current wireless link 950 after having beencommunicated from BS2 903 sector 906 through network link 924 to BS1 901sector 904. Different segments may use different sets of tone-symbols.In some embodiments, different types of access segments use differentsets of tones. In some embodiments, as shown in FIG. 8, the tone-symbolsof a segment are contiguous; however, in other embodiments, tone-symbolsincluded in a segment may not be continuous.

In some embodiments of the invention, a sector uses differentfrequencies to transmit the beacon signal into the adjacent sectorduring a set of N successive beacon slots. The N successive beaconslotsform what is referred to as an ultraslot. The exact pattern of beaconsignaling does not repeat in the exemplary embodiment within theultraslot, e.g., different beaconslots may use slightly differentfrequencies for beacon tones, but does repeat with the next ultraslot.However, the beacon signaling pattern will repeat from one ultralslot tothe next.

From a beacon signal received from an adjoining base station sector, amobile device 902 can, and in various embodiments does one or more ofthe following: determine the quality of the communications channelbetween the mobile device and the base station sector from which thebeacon signal was transmitted and select between a plurality of sectorsto make a handoff decision based on beacon signal measurement(s) andother information such as traffic loading, determine the cell and/orsector identifier, e.g., slope, of the cell which includes thetransmitting sector, determine the frequency band (e.g., sector type) ofthe sector and/or sector carrier corresponding to the transmitted beaconsignal, determine the relative timing within a super slot between thetiming in the current base station sector and the timing within asuper-slot of a base station sector selected by the mobile node to bethe new base station sector to which a handoff operation is to becompleted.

Once a decision to implement a handoff is made by a mobile device, e.g.,based on the relative strength of beacon signals received from differentnetwork attachment points, in accordance with the present invention, ahandoff is initiated by the mobile device through the current basestation sector with which the mobile device is communicating. In thismanner a handoff can be initiated through the current base stationsector without the need for the mobile device to switch itsreceiver/transmitter circuitry from the frequency band of the currentbase station sector to the frequency band of the new base stationsector. FIG. 10 illustrates various exemplary handoff related signaling,e.g., which occurs in some embodiments. The mobile device 902 transmitsa signal 1002 to the current base station sector 904, e.g., a cellidentifier and/or a sector type identifier corresponding to the adjacentsector 906 to which a handoff is to be completed. The current networkattachment point, base station sector 904 in this example, uses thisinformation to enable communications between the mobile node 902 and thenew network attachment point, e.g., base station sector 906 in thisexample. In some embodiments the current BS sector acts as a router andsimply relays handoff messages between the mobile node and the newnetwork attachment point. However, for communications purposes and toreduce the amount of signaling required over the current air link withthe mobile device, the current base station sector 904 may, and oftendoes, act as a proxy in either relaying communications between themobile device 902 and the new base station sector 906 or negotiatinghandoff with the new base station sector 906 on behalf of the mobiledevice 902. Thus, handoff information is communicated on behalf of themobile node to the new base station sector over links, which are oftenwireline (e.g., copper or fiber optic lines), connecting base stationsand/or sectors within a base station. In the case of links between basestations, such links may include backhaul links. Handoff communicationsbetween the base station sectors may, and often are, subject toauthentication and/or other security procedures, including encryption,before handoff communication is allowed to proceed further. In suchembodiments, a secure communications link is established between thecurrent network point of attachment and the new network point ofattachment with handoff messages including resource assignments beingpassed over the secure link.

In FIG. 10, signal 1004 represents the transmission of a signal from BS1to the new BS sector 906 to initiate a handoff on behalf of mobile 902.This signaling may include mobile node identification information aswell as the base station identifier and sector identifier supplied bythe mobile 902 and/or other information indicating an intent to initiatea handoff. The new BS sector 906 responds by sending a securitychallenge 1006 to the current BS sector 904. The BS sector 904 respondswith the correct reply 1008 thereby establishing a secure communicationslink for purposes of further handoff related signaling. In analternative embodiment, at least some of the above steps of 1004, 1006and 1008 are omitted. The WT sends information (see below step 1010)through the current base station sector to the new base station sector.

Once a sufficient level of security is established between the currentbase station sector 904 and the new base station sector 906, the mobilenode 902 can communicate through the current base station sector 904information including its intent to complete a handoff to the new basestation sector 906 and/or receive information from the new BS sector906. Following the current base station sector 904 signaling the newbase station sector 906 of the impending mobile device handoff, the newbase station sector 906 assigns the mobile device 902 a dedicatedcommunications resource, e.g., at least one device identifier to be usedby the mobile device 902 in regard to air link signaling upon entry intothe new sector 906. In some systems multiple device identifiers areassigned to the mobile 902 for use in a sector 906, e.g., one to be usedby the mobile when operating in an “on-state”, and another identifieridentifying the mobile among a set which includes a relatively largenumber of mobiles which can operate in the cell in a “hold” state at thesame time in the sector. Signal 1010 represents the transmission ofdevice identifiers and resource allocation information to the WT 902 viathe current BS sector 904. Thus, device identifier assignments, used forphysical layer signaling such as over the air signaling, are made by thenew base station sector 906 to the mobile 902 via the current basestation sector 904. In addition to assigning device identifiers to beused in the new base station sector, the new base station sector 906can, and often does, reserve dedicated resources, e.g., uplink and/ordownlink channel segments, for the mobile device for purposes ofperforming access including initial closed-loop power control and/ortiming control signaling upon entry into the cell, e.g., as part of aregistration process. In various embodiments, a set of tone-symbols,e.g., segment 844, dedicated for timing control and/or a set oftone-symbols, e.g., segment 856, dedicated for power control signalingpurposes during registration is assigned to the mobile device 902 by thenew base station sector 906. Each set of dedicated tone-symbols, e.g.,segment 844, may be one of a plurality of sets of tone-symbols, e.g.,segments, available in a particular portion of the air link resources,e.g. available to handoffs but not to new initial entry into the cell.Those tone-symbol sets, e.g., segment 844, are used on the basis of theassignment given by the base station. Therefore, although thosetone-symbol sets, e.g., segments, are used for access, there is nocontention in those tone-symbol sets, e.g., segments, since they arededicated for use to a specific WT. Moreover, as the use of thetone-symbol sets, e.g., segments, is based on the assignment, the basestation knows which mobile devices are supposed to use which tone-symbolsets, e.g., segments, which is very different from a contention-basedrandom access situation where the base station does not necessarily knowthe identity of the mobile devices even after the base station hasdetected one or more access signals. In signal 1010, which may includemultiple IP packets and/or separate messages, the assignment ofdedicated resources to be used to complete timing and/or power controlupon entry into the new base station sector is communicated to themobile device along with information identifying the time period withinan ultra slot in which the resources are dedicated to the mobile, e.g.,the timing of the dedicated uplink segment in the ultraslot. The timeperiod is specified, in some embodiments, within an ultraslot 818 totake into consideration the fact that communications between the mobilenode 902 in the current base station sector 904 and the new base stationsector 906 via the current base station sector 904 may take longer thana superslot time period to communicate due to communications delaysassociated between the links between base station sectors 904, 906. Themobile device, e.g., WT in some embodiments interprets the assignmentinformation using stored information about the framing structure of thecommunications channels used by the new network attachment point. Thisinformation may, and in some embodiments is, accessed and retrievedusing beacon information. For example, a WT may retrieve from memorycommunications channel information relating to a network access pointcorresponding to a beacon signal which resulted in the network accesspoint being selected as the new network access point. This informationcan be used to interpret resource assignment information received fromthe new network access point and/or to determine the time of a dedicatedsegment relative to the time a beacon signal was received.

In addition to the dedicated resources, e.g., tone-symbol sets, e.g.,segments, set aside in a particular access slot for a pre-assignedmobile node which has signaled an intent to transfer to the new basestation sector, other tone-symbol sets, e.g., other segments, areavailable in some embodiments for use on a contention basis, e.g., formobile nodes newly entering the cell without prior notification viaanother base station sector to performing timing and power controloperations. FIG. 8 illustrates such contention based segments that areset aside during each access slot. During access slot 858 four exemplarycontention based segments are set aside as indicated by segments 872.Similarly, during subsequent access slots, sets of contention basedsegments 874, 876, 878, 880, 882, 884, 886, and 888 are available. Eachset of four segments, e.g., segment set 872 may accommodate two WTs,where each WT uses one segment for timing synchronization operations andone segment for power control operations. In some embodiments, the WTuses the contention based access segments following a failure of ahandoff attempt using allocated dedicated access segments.

By using dedicated pre-assigned resources during the access(registration) interval, e.g., registration slot, as opposed to tryingto use resources where collisions are possible, e.g., due to competingdevices attempting to use the same set of tones at the same time, thechance a mobile node entering the base station sector and being able tocomplete registration, timing control, and/or power control operationsat a predictable time, e.g., at a particular time within an ultraslot,is greatly increased as compared to where contention based resourceallocation is used.

Upon entry into a base station sector from another cell, a mobile nodemay be required to complete timing synchronization and/or power controlsignaling before being allowed to receive/send IP packets correspondingto communications sessions via the new base station sector. The handoffmethods of the present invention increase the predictability of whensuch IP signaling will occur while reducing the time required tocomplete physical layer power control and timing synchronizationoperations upon entry into a new base station sector.

In accordance with one feature of the present invention, an IP routingupdate signal 1012 is sent, via the current base station sector 904, tocause IP packets intended for the mobile node 902 to be redirected tothe new base station sector 906, after a handoff operation is initiated.This normally occurs before handoff signaling has been completed in thenew sector, e.g., before registration, power control signaling, and/ortiming control signaling required for the WT to receive/send packets viathe new link has been completed. The signaling 1012 may be to a mobileIP home agent 914 responsible for redirecting packets addressed to themobile node 902 to the mobile node's current point of networkattachment. Given delays associated with the communication of suchrouting update signals, by initiating the IP packet redirection from thecurrent base station before physical layer signaling setup operationsare completed in the new base station sector, packet redirection delayscan be made to correspond to the time period during which the mobiledevice 902 is temporarily unreachable due to the delays associated withfrequency band switching, timing synchronization operations and/or powercontrol signaling. Thus, by the time the mobile device 902 is able toreceive IP packets in the new base station sector 906, or shortlythereafter, the IP routing update operation may have been completed.

In some embodiments, the IP routing update request 1012 is transmittedin response to the mobile device 902 being assigned a resource, e.g., anidentifier, to be used in the new base station sector 906 and/or beingassigned dedicated communications resources required to complete anytiming control and/or power control operations which need to becompleted prior to the mobile device 902 being able to receiver IPpackets in the new base station sector 906. In such embodiments, IProuting updates 1012 are transmitted via the current base station sector904 after it is known with a high degree of certainty that a handoffoperation will successfully be completed. The routing update message maybe triggered by receipt of a resource assignment message from BS2, atBS1, which is directed to the WT seeking to complete a handoff. In suchcases, IP routing updates will not be triggered in cases where the newbase station sector 906 is unable to allocate the resources required toaccept the mobile device 902, e.g., because the maximum supported numberof devices are already present and active in the cell precluding deviceID assignment. In cases where packets are received at the new BS priorto the WT establishing a communications link with the new BS, the BSstores the received packets in a buffer and, upon successful completionof a handoff and establishment of a communications link supplies thepackets addressed to the WT to the WT over the newly establishedcommunications link.

A handoff initiated via a current base station sector 904 may fail to becompleted successfully, e.g., due to interference with the dedicated setof tone-symbols allocated for initial timing and/or power controloperations with regard to the new base station sector 906. In somecases, the above described handoff process is repeated but this requiresconnectivity to be re-established through the old BS sector 904.However, in other cases rather than attempt to initiate a handoff viathe current base station sector 904 again, having already switched tothe frequency sub band of the new base station sector 906 and therebyterminating the communications link via the old sector, the mobile node902 proceeds to register in the cell in the same manner as other mobiledevices entering the cell without having a pre-existing communicationssession with an adjacent sector. In such embodiments, if a registrationis not successfully completed using the dedicated set of resourcesassigned to a device as part of a handoff operation, the base stationsector 906 frees the airlink resources dedicated to the WT seeking toimplement a handoff e.g., the assigned mobile device identifier isreleased for use by another device.

After successful registration by a mobile node with a new sector, thenew BS sector 906 becomes the current BS sector through which IP packetsare to be communicated between the mobile device 902 and other devices.Signaling 1014 represents the transmission of radio signals to the newBS sector 906 via wireless link 952 to communicate IP packets followingsuccessful registration.

FIG. 11 is a flowchart 1100 of an exemplary method of operating awireless communication system, e.g., an OFDM wireless communicationssystem using beacon signals, to perform handoffs of wireless terminalsfrom one base station sector attachment point (AP) to another basestation sector attachment point in accordance with the presentinvention. In the case were a single carrier is used in each sector, thesector servers as the base station sector attachment point. The steps inFIG. 11 refer to BS sectors. These references are to be interpreted asreferring to BS sector attachment points which, in the case of a singlecarrier BS sector, will in fact be the same as the BS sector. However,when multiple carriers are used in a sector, the sector may includemultiple BS sector attachment points, one for each of the carrierssupported in the sector. In an embodiment which supports multiple BSsector attachment points per sector, each BS attachment pointcorresponding to a different carrier, the receiver componentscorresponding to each carrier serve as a separate base stationattachment point. In such an embodiment a handoff can occur within asector from one carrier to another carrier frequency as the attachmentchanges from a BS attachment point associated with one carrier to a BSattachment point in the same sector corresponding to another carrier.Operation starts in step 1 102, in which an exemplary WT is currentlyattached to a base station sector attachment point.

For purposes of explaining the method of the invention, it will beassumed that the base stations in the system are transmitting beaconsignals on a periodic basis for each of the possible BS sectorattachment points and the WT has strength information on the lastreceived beacon signal corresponding to the current attachment point.FIGS. 3 and 4 are exemplary of the type of signaling which may occur inmulti-sector cells with one carrier pre-sector which each sector servesas a single point of network attachment.

Operation proceeds from step 1102 to step 1104. In step 1104, the WTmonitors for beacon signals. Detected beacon signals are identified asto their transmission source, e.g., corresponding base station sectorand corresponding carrier frequency, measured as to their receivedsignal strength level, and the obtained information is stored.

Then, in step 1106, for each detected beacon a comparison is performedto determine if a beacon signal of a potential carrier corresponding toa different sector and/or carrier within the current sector is strongerthan the current BS sector attachment point beacon signal. If thepotential carrier BS sector beacon signal is not stronger than thecurrent BS beacon signal, then operation returns to step 1104, where theWT continues to monitor for additional beacon signals. However, if adetected adjacent BS sector beacon signal is stronger than the currentBS sector beacon signal, then operation proceeds to step 1108. In step1108, the WT is operated to check if handoff criteria are satisfied. Forexample, satisfied handoff criteria may include the potential carrier BSbeacon signal being stronger than the current BS beacon signal by apredetermined margin, the potential carrier BS beacon signal meeting aminimal signal strength threshold level, and/or the potential carrier BSbeacon signal having exceeded the current BS beacon signal for apredetermined amount of time or number of successive iterations. If thehandoff criteria of step 1108 is not satisfied, operation proceeds fromstep 1108 to step 1104, where the WT continues to monitor for additionalbeacon signals. If the handoff criteria of step 1108 is satisfied,operation proceeds to step 1112.

In step 1112, the WT determines the cell ID, sector ID, and otheridentifying information, e.g., carrier frequency, of the new BS sectorattachment point, e.g., new section and/or current section but newcarrier frequency selected for the handoff. Then in step 1114, the WT isoperated to signal the current BS sector to initiate a handoff to thenew BS sector. The new BS sector requested attachment point may be,e.g., in a different cell, in a different sector of the same cell, or inthe same sector of the same cell using a different carrier frequency.Operation proceeds from step 1114 to step 1116. In step 1116, thecurrent BS sector attachment point initiates secure communicationsthrough the network to the new BS sector attachment point. In the caseof an inter-cell handoff, a secure communications link is establishedbetween the two base stations, e.g., through the backhaul network links,and the request from the WT is forwarded over the secure link from thecurrent base station sector to the new BS sector. In the case of anintra-cell or intra-sector handoff signaling is internal to the BS andmay therefore be secure by the physically limited nature of the link.Operation proceeds from step 1116 to step 1118. In step 1118, the new BSsector allocates an air link resource to the WT, e.g., assigns anon-state identifier and/or a hold-state identifier to the WT, reservesadditional resources for the WT such as an uplink transmission segment,and transmits information including access state in which the new basestation now has reserved a dedicated tone set, e.g., dedicated tones tobe used during the time period of the dedicated uplink segment, forregistration by the WT. In some embodiments, reserved resources includededicated uplink timing control channel segments, dedicated uplink powercontrol channel segments, and/or dedicated uplink traffic channelsegments. In some embodiments, each type of channel uses different setsof tones. In intra-cell handoffs, an initial dedicated uplink timingcontrol channel segment may not be required and may not be reserved andassigned, as the new BS sector point of attachment being collocated withthe current base station sector and sharing common clock circuitry maybe operated to be timing synchronized with respect to the current basestation sector, allowing the WT to skip an initial timingre-synchronization step in the registration process of the handoff.Operation proceeds from step 1118 to step 1120. In step 1120, the WTterminates wireless signaling with the old BS sector attachment point,e.g., by ceasing to transmit addition signals on the uplink over theoriginal wireless link. The point in time chosen to terminate theoriginal wireless link is determined by the WT to be prior to thetransmission of the earliest uplink signaling to the new BS sectorattachment point using the allocated dedicated resources, e.g., justprior to the uplink timing control signaling to the new BS sector usingthe allocated dedicated segment or at some fixed time prior to uplinksignaling to the new attachment point. At this point in time, or shortlybefore the point the connection is terminated, the current BS sector instep 1121 may transmit a routing update message signaling the IP routingsystem to start routing packets including an address corresponding tothe WT to send the IP packets to the new BS even though the registrationwith the new BS has not been completed. Operation proceeds from step1121 to step 1122 The dedicated segments assigned by the new BS sectorhave a fixed timing relationship to the beacon signal corresponding tonew BS sector, and this known relationship can be used by the WT in thedetermination of original link termination time. Operation proceeds fromstep 1120 to step 1122. In step 1122, the WT adjusts its receiver to thefrequency band of the new BS sector attachment point. Then, in step1124, the WT registers with the new BS sector attachment point using thededicated resources, e.g., assigned identifier, dedicated uplink channelsegments including dedicated tone sets in specified access slot(s). Inthe case of an inter-cell handoff this involves transmitting timingcontrol and/or power control signals to the new BS sector before userdata is transmitted to the new BS sector. A timing control signal to theBS, in some but not all embodiments, is used for multiple purposes andcan serve, e.g., as a registration signal in addition to serving as atiming control signal. In the case of an intra-cell handoff the timingcontrol signaling operation is skipped in some embodiments since timingsynchronization is maintained in some embodiments across sectors of thecell. Power control signaling is optional and need not be performed inall intra-cell and inter-cell handoffs before the WT can receive andtransmit user data. The BS sector responds to the timing and/or powercontrol signals, when used as part of the registration process, bytransmitting corresponding control signals to the WT. A timingsynchronization (control) signal is transmitted to the WT in response toa received timing control signal. The timing synchronization signal canindicate to the WT that it should advance, retard or leave itstransmission timing unchanged. In the case of power control signaling, apower control signal is transmitted to instruct the WT to, e.g.,increase, decrease or leave its transmission power unchanged.

In the case of an inter-cell handoff operation proceeds from step 1124to step 1125 wherein the WT adjusts its transmission timing in responseto the timing synchronization signal received from the new BS. In thecase of intra-cell handoffs the initial timing control performed in step1125 as part of the handoff can be skipped when symbol timingsynchronization is maintained throughout the sectors of a cell andalready exits with the WT as a result of one or more previous symboltransmission timing adjustments made based on one or more timing controlsignals received from the BS sector to which the WT was attached priorto the handoff. Operation proceeds from to step 1125 to step 1126wherein the WT adjusts its transmission power, assuming transmissionpower control is performed as part of the registration process inresponse to a transmission power control signal received from the newBS. In various embodiments, the transmission power control is optionalin the registration process. Accordingly, in some embodiments step 1126is skipped. Operation proceeds from step 1126 (or 1125 when step 1126 isskipped) to step 1127 where the new BS sector checks to determinewhether or not the registration was successful. For example, the new BSsector attachment point checks if it has successfully receivedregistration information from the WT over the dedicated assigned uplinksegments during the assigned access slot, e.g., receiving the properidentifier and signaling to achieve WT timing synchronization and WTpower control signaling when implemented. If the registration wassuccessful, operation proceeds from step 1126 to step 1132. In step1132, the new BS sector attachment point becomes the WT's point ofnetwork attachment at which point the WT can begin transmitting userdata, e.g., text, voice and/or image data included in IP packets, to theBS. The new BS sector attachment point can also begin transmitting IPpackets directed to the WT over the established communications link. Asa result of the routing update process which begins in some embodimentsof the invention prior to the registration process with the new BSsector attachment point being completed, packets addressed to the WT maybegin being received by the new BS sector attachment point, e.g., new BSsector in the case of a single carrier sector, prior to completion ofthe registration process. Such packets are temporarily stored andforwarded to the WT over the new communications link upon completion ofthe registration process in step 1132 with the new BS sector attachmentpoint now serving as the WT's network point of attachment. Operationproceeds from step 1132 to step 1104, where the WT monitors foradditional beacon signals. Returning to step 1126, if the registrationwas not successful, e.g., the new BS sector attachment point was notable to obtain appropriate registration information and signals, e.g.,due to interference, then operation proceeds to step 1128. In step 1128,the new BS sector attachment point releases the assigned IDs andassigned dedicated resources, e.g., dedicated uplink segments. Operationproceeds from step 1128 to step 1130. In step 1130, the WT registerswith the new BS sector attachment point as a new WT entering the new BSsector would, e.g., using contention based uplink resources to requestregistration with the BS sector. Operation proceeds from step 1130 tostep 1132, in which the new BS sector attachment point becomes the WT'spoint of network attachment.

FIG. 12 is a flowchart 1200 of an exemplary method of operating a mobilecommunications device, e.g., a mobile wireless terminal such as a mobilenode, to implement a handoff of the mobile communications device betweena first base station and a second base station, said mobilecommunications device having a first wireless communications link withthe first base station at the time said handoff is initiated. The methodof implementing the handoff starts in step 1202 and proceeds to step1204. In step 1204, the mobile communications device is operated toreceive a signal, e.g., a beacon signal, from said second base station,said signal having known timing offsets from uplink channel segmentswhich may be dedicated to said mobile communications device. The firstand second base stations may not be synchronized with respect to oneanother, and the mobile communications device, operating with respect tothe first base station timing can and does advantageously use the secondbase station beacon signal to determine timing associated with secondbase station dedicated uplink segments. Operation proceeds from step1204 to step 1206. In step 1206, the mobile communications device isoperated to signal to said second base station, via a first signalcommunicated over said first link, an intent to initiate a handoff tosaid second base station. For example, the mobile communications devicecan send a request for handoff over the first wireless communicationslink, and the first base station can forward the request via thebackhaul network to the second base station. Operation proceeds fromstep 1206 to step 1208. In step 1208, the mobile communications deviceis operated to receive from the second base station, via a second signalcommunicated via said first link, information indicating a resource(s)dedicated to said mobile communications device by said second basestation to be used in communicating with said second base station. Forexample, the second base station may send information indicatingdedicated resources, thus conveying a grant corresponding to the handoffrequest. The information may be conveyed from the second base station tothe first base station via the backhaul link, and the first base stationmay forward such information as a second signal over the first wirelesslink. The dedicated resource(s) may be, e.g., an uplink timing controlsegment, an uplink power control segment, an uplink traffic channelsegment, and/or a base station specific wireless terminal identifierdedicated to said mobile communications device by said second basestation to be used in communications with said second base station.Operation proceeds from step 1208 to step 1210.

In step 1210, the mobile communications device is operated to terminatesaid first communications link. For example, the termination may beperformed by the mobile communications device ceasing communicationsover the first communications link. Step 1210 includes sub-steps 1212and 1214. In sub-step 1212, the mobile communications device is operatedto determine the time to terminate said first link based upon saidreceived signal, e.g., a received beacon signal, from the second basestation and indicated dedicated uplink channel segment(s). For example,the mobile communications device may determine the termination time asthe point in time just prior to the time of the earliest indicateddedicated uplink segment to be used by said mobile communications deviceto send signals to the second base station over the second wirelesslink, e.g., the time of the assigned dedicated timing control uplinksegment. Operation proceeds from sub-step 1212 to sub-step 1214. Insub-step 1214, the mobile communications device is operated to terminatethe first link at the time determined in sub-step 1212 based on a signalreceived from the new BS. In some embodiments, the termination mayinvolve sending a termination signal from the mobile communicationsdevice to the first base station over the first wireless link. In someembodiments, the mobile communications device terminates the firstwireless communications link by ceasing to send additional signalingover the link. Operation proceeds from step 1210 to step 1216.

In step 1216, the mobile communications device is operated to use saiddedicated communications resource(s) to communicate via a secondwireless communications link with said second base station. For example,the mobile communications device may have been assigned by the secondbase station an identifier to be used in wireless communications withthe second base station over the second communications link. In someembodiments, some specific dedicated uplink segments may be associatedwith a specific identifier and reserved for use by the mobilecommunications device assigned by the base station to use that specificidentifier. In some embodiments, some dedicated uplink segments areassigned by the base station on a segment by segment basis to mobilecommunications devices. Step 1216 includes sub-steps 1218, 1220 and1222. In sub-step 1218, the mobile communications device uses adedicated resource, e.g., an assigned uplink timing control segment, toperform a timing control synchronization operation. For example, themobile communications device sends uplink signaling during the assigneduplink timing control segment, and the signaling is received by thesecond base station. A signal received from the BS is then and used tosynchronize timing between the mobile communications device and thesecond base station. The timing synchronization operation normallyinvolves adjusting the WTs symbol transmission timing based on a signalreceived from the BS. Operation proceeds from sub-step 1218 to sub-step1220. In sub-step 1220, the mobile communications device is operated touse said dedicated resource, e.g., an assigned uplink power controlsegment, to perform a power control operation. For example, the mobilecommunications device sends, using an assigned uplink power control orother segment, a signal at a specified power level to be received andmeasured by the second base station. The base station subsequentlyconvey power adjustment signals to the mobile communications device towhich the mobile responds by adjusting its transmission power level.Operation proceeds from sub-step 1220 to sub-step 1222. In sub-step1222, the mobile communications device is operated to transmit user datae.g., voice, text, or other information, over the second communicationslink that has been established with the second base station. User datacan be communicated using one or more dedicated uplink traffic segmentsmay have been assigned by the second base station to the mobilecommunications device, and the mobile communications device, which hasbeen previously timing synchronized and power controlled based onsignals from the new base station which can communicate user data in areliable manner on the uplink to the second base station.

FIG. 13 is a flowchart 1300 of an exemplary method of operating a mobilenode to implement a handoff between a first link with a first basestation sector and using a first carrier and a second link with a secondbase station sector, said second link using a second carrier, at leastthe first sector being different from the second sector or the secondcarrier being different from the first carrier. For example, thisexemplary method may be used for intra-cell inter-sector handoffs of amobile node where the carrier used is the same or different. Theexemplary method may also be used for intra-cell intra-sectorinter-carrier handoffs of a mobile node. The exemplary method ofimplementing the handoff starts in step 1302 and proceeds to step 1304.In step 1304, the mobile node is operated to receive a timing correctionsignal over said first communications link, e.g., as part of the normaltiming control process performed when operating in a cell. Operationproceeds from step 1304 to step 1306. In step 1306, the mobile node isoperated to make a transmission timing adjustment to adjust the timingof signals, e.g., symbols, transmitted by said mobile node over thefirst link. Then, in step 1308, the mobile node is operated to signal anintent to handoff to the second link. For example, the mobile node maysend a handoff request signal over the first link to the first basestation sector point of attachment, and the handoff request may beforwarded to the second base station sector point of attachment, wherethe first and second base station sectors may be different sectors ofthe same base station. Alternatively, where an intra-sectorinter-carrier handoff is being performed, the signal may be sent via amodule corresponding to the first carrier in the first sector to amodule corresponding to the second carrier in the first sector, in sucha case the first and second sector are the same but the carriers usedare different. The second base station sector point of attachment maygrant the handoff request and respond by assigning some dedicatedresources to the mobile node and conveying information identifying thoseassigned dedicated resources to the mobile node via the first basestation sector point of attachment and its wireless link, the firstlink. Operation proceeds from step 1308 to step 1310. In step 1310, themobile node is operated to receive over said first communications linkinformation indicating a dedicated resource to be used by said mobilenode when communicating over the second communications link. Dedicatedresources may include, e.g., an identifier specific to said secondsector and said second carrier, a dedicated uplink power controlsegment, and/or a dedicated uplink traffic channel segment. Operationproceeds from step 1310 to step 1312. In step 1312, the mobile node isoperated to terminate said first communications link. In someembodiments, the mobile node terminates the first communications link bysending a termination message to the first base station sector. In someembodiments, the mobile node terminates the first communications link beceasing to send additional signaling over the first communications link.The mobile node can advantageously terminate the first communicationslink at a point in time prior, e.g., just prior, to utilizing theearliest dedicated uplink segment, e.g., a dedicated uplink powercontrol segment or a dedicated uplink traffic channel segment, assignedby the second base station sector to the mobile node. Operation proceedsfrom step 1312 to step 1314. In step 1314, the mobile node is operatedto transmit at least one of user data and a non-timing control signal,e.g., a power control signal, over said second communications link priorto receiving a timing control signal over said second communicationslink. This is done, in some embodiments, prior to altering thetransmission timing based on a signal received from the new BS sectorpoint of attachment following termination of the first link. Forexample, the first and second base station sectors, being part of thesame base station, allow for synchronization between the sectors thusallowing the mobile node to maintain timing synchronization as a handoffbetween sectors occurs; this allows the timing synchronization stepsnormally required in an inter-cell handoff operation to be omitted insome embodiments minimizing the overhead control signaling involved inan intra-cell handoff and providing for quicker intra-cell handoffs withshorter interruptions in operation.

In some embodiments, intra-sector and inter-sector handoff embodiments,the mobile node uses dedicated air link resources for communicationsduring a time interval extended from said point in time when the mobileterminates said first communications link to a point in time where themobile transmits user data over said second communications link, saidmobile node avoiding the use of shared communication resources whichother mobile nodes can access at the same time as said mobile nodeduring the time interval. By utilizing dedicated resources for controlsignaling, e.g., power control, during this time interval in the handoffoperation and not utilizing shared resources, collisions between usersresulting in the disruption of a smooth handoff and the associated lossof time and repetition of steps may be avoided resulting in moreconsistent and efficient handoffs than would be possible if sharedresources were used.

FIG. 14 is a flowchart 1400 illustrating an exemplary method ofoperating a base station sector to implement handoffs between basestation sectors and/or between carrier in a sector corresponding todifferent points of attachment in accordance with the methods of thepresent invention. As can be appreciated, control circuitry or modulesassociated with different carriers in a sector or different sectors canoperate as different points of attachment. Operation of the exemplarymethod starts in step 1402 and proceeds to steps 1404, 1406 and 1408.

In step 1404, the base station sector is operated to generate andperiodically broadcast signals, e.g., beacon signals, said broadcastsignals having a fixed timing relationship to dedicated uplink channelsegments used in handoffs associated with the BS sector from which thebeacon signal originates.

In step 1406, the BS sector is operated to receive signals over itswireless interface, e.g., sector receive antenna and sector receiver.Operation proceeds from step 1406 to step 1410. In step 1410, the BSsector operation is determined based on the type of signal received. Ifthe received signal of step 1406 was a handoff request to another BSsector as illustrated in block 1412, then operation proceeds to step1414. If the received signal of step 1406 was a timing control signalusing a dedicated resource as illustrated in block 1426, then operationproceeds to step 1428. In step 1428, the BS sector processes thereceived timing control information, e.g., establishing timingsynchronization as part of handoff operations in establishing a newwireless link. If the received signal of step 1406 was a power controlsignal using dedicated resources as illustrated in block 1430, thenoperation proceeds to step 1432. In step 1432, the BS sector processesthe power control information received, e.g., performing WT powercontrol signaling as part of handoff operations in establishing a newwireless link. In some embodiments, the BS sector will not process WTpower control signals in step 1432 if the timing control processing ofstep 1428 was required and has not been previously successfullyperformed. If the received signal of step 1406 was user datacommunicated using dedicated resources as illustrated in block 1434,then operation proceeds to step 1436. In step 1436, the BS sectorprocesses the user data received, e.g., forwarding the user data towardanother WT. In some embodiments, the BS sector will not process userdata signals in step 1436 if the timing control processing of step 1428and/or power control processing of step 1432 were required and have notbeen previously successfully performed.

Returning to step 1414, which relates to a handoff between differentpoints of attachment, the BS is operated to determine the type ofhandoff and direct operations based on whether the hand-off request wasan inter-cell or an intra-cell handoff request. If the request was anintra-cell request, e.g., an intra-cell inter-sector or intra-cellinter-carrier handoff request, then operation proceeds to step 1416;however, if the request is an inter-cell handoff request then operationproceeds to step 1418. In step 1416, the BS sector point of attachmentis operated to forward the request to the requested BS sector point ofattachment, e.g., an adjacent sector within the same BS or circuitrycorresponding to a different carrier in the same sector. From step 1416,operation proceeds to step 1420. In step 1420, the BS sector is operatedto receive information from the new (requested) BS sector indicatingdedicated resources, e.g., identifiers and/or dedicated segments forhandoff operations, e.g., dedicated uplink power control channelsegments within a specified access slot. In some embodiments, thededicated resources of step 1420 does not include uplink timing controlchannel segments during an access slot, as the base stations sectorswithin a given BS in some embodiments are timing synchronized withrespect to one another. Operation proceeds from step 1420 to step 1422.In step 1422 the BS sector is operated to convey dedicated resourceinformation over the original established wireless link to therequesting WT.

Returning to step 1418, in step 1418, the BS sector point of attachmentis operated to forward some information to the new requested BS sectorpoint of attachment indicating a handoff request. Operation proceedsfrom step 1418 to step 1424. In step 1424, the BS sector is operated toestablish a secure BS-BS link. Once the secure link is established,detailed information regarding the handoff can be conveyed between thenew requested BS sector point of attachment and the WT via the oldexisting BS sector using the backhaul network and the existingestablished wireless link.

Returning to step 1408, in step 1408 which relates to inter-cellhandoffs, the BS sector is operated to receive signals via its networkinterface. Operation proceeds from step 1408 to step 1411, where the BSsector is operated based on the type of signal received. If the signalreceived in step 1408 is information indicating a handoff request to theBS sector 1438, operation proceeds to step 1440, where the BS sector isoperated to establish a secure BS-BS link. Operation proceeds from step1440 to step 1442. In step 1442, the BS sector is operated to dedicateresources, e.g., identifiers and/or dedicated segments such as uplinktiming control channel dedicated segments and uplink power controlchannel dedicated segments during a specified access slot by the WT forhandoff operations. In some embodiments, the dedicated segments of step1442 include dedicated uplink traffic channel segments to be used by theWT after timing and power control has been established. In step 1442,the BS sector is also operated to signal information identifying thosededicated resources to the other BS over the secure BS-BS link.

If the received information in step 1408 is information indicatingdedicated resources, e.g., identifiers and/or dedicated segments to beused by the WT for handoff operation, then operations proceed from step1411 to step 1446. In step 1446 the BS sector conveys the receiveddedicated resource identification information over the originalestablished wireless link to the requesting WT.

It should be noted that the above described intra-cell and inter cellhandoff methods may, and in some embodiments are, used sequentially. Forexample, an intra-cell handoff method of the invention can be used tohandoff from one sector to another in a cell, one or more times before aWT implements a handoff from one cell to another using an inter-cellhandoff method of the invention. In the case of the intra-cell handoffs,user data may be transmitted upon entry into the new sector or using anew carrier in the cell prior to making a timing adjustment in responseto a signal received over the air following termination of the oldcommunications link. However, when the inter-cell handoff occurs, the WTwill normally perform a timing synchronization operation, e.g.,adjusting its symbol transmission timing, based on one or more signalsreceived over the air from a transmitter in the new cell, prior totransmitting user data in the new cell.

Numerous different system and handoff method implementations arepossible using the methods and apparatus of the invention.

For example, in one exemplary system with multiple frequency bands, amobile listens to one frequency band at a time, converts (e.g., byperforming an FFT or DFT) received signals from the time domain to thefrequency domain, measures the energy on each of the signal componentsin the frequency domain generated by the frequency transform operation,(e.g., the per tone signal energy); and detects the presence of beaconsignal components based on the received per tone signal energy. In thisparticular exemplary embodiment, from the locations of beacon tones themobile determines cell/sector and/or carrier info (e.g., a cell ID,sector Id and/or carrier frequency) of the base station transmitter thattransmitted the beacons. From the energy of beacon signal components,e.g., detected beacon signal tones, the mobile then determines therelative signal strengths of various base station transmitters whichtransmitted beacon signals into the frequency band being used by themobile node. From the relative energy of the beacon signal componentsreceived from different transmitters, and various handoff criteriainformation stored in the mobile, the mobile determines whether ahandoff should be implemented and, in the case of a decision toimplement a handoff, the new base station attachment point,corresponding to a transmitter from which a beacon signal was received,to handoff to. In some implementations of such an exemplary system, abeacon signal component is identified based on comparing the signalcomponent energy level to an average per tone energy level. In someembodiments, where the beacon power >20 times the average per tonesignal power over a time period such as 1 or 2 seconds, the beacondetection threshold is set at or slightly below 20 times the detected oranticipated average per tone energy, e.g., at 15 times the expectedaverage per tone signal energy.

Once a system decides to implement a handoff various handoff methods ofthe invention may be used. The handoff techniques described herein arenot dependent on the particular method of deciding when implement ahandoff. However, diction of one or more beacon signals is used invarious handoff embodiments to determine timing and/or other informationrelating to the network attachment point, e.g., BS sector, to which amobile node seeks to complete a handoff.

While inter-cell and intra-cell handoffs have many common steps andfeatures when performed in accordance with various embodiments of theinvention, inter-cell handoffs may involve timing synchronization and/orpower control steps which may be required to be performed before amobile node is permitted to transmit user data, e.g., text, video orvoice data to the new network point of attachment. This is because, inthe case of intra-cell handoffs, where the various elements such assectors are synchronized, the mobile node can rely on previouslyestablished timing synchronization with an access point in the cellwhich should remain reasonably reliable given timing synchronization inthe cell even though the mobile changes the point of network attachmentbeing used within the cell.

From a base station perspective, some features that can be used supportboth intra-cell and inter-cell handoffs are such things as the use ofmultiple frequency bands, the transmission of beacon signals by anetwork attachment point into the frequency band used by the networkattachment point to communicate data and into the frequency band beingused in a neighboring sector, cell or network attachment point for thecommunication of user data. Thus, a network attachment point transmitterwill normally transmit beacon signals into multiple frequency bands. Tofacilitate interpretation of airlink resource assignments and theprocesses of determining when to terminate an existing communicationslink as part of a handoff, a fixed framing structure is used for theuplink and/or downlink supported by each network attachment point.Traffic, access and other types of segments which are used tocommunicate specific types of data and/or for specific purposes repeatin a predictable known manner given the fixed communication channelframing structure. As a result, once the time location of a receivedbeacon signal is known, the communications channel structure, e.g.,superslot/beaconslot structure of the frequency band in which thetransmitter transmits its user data (traffic channels, etc) can beuniquely derived from the beacon signal location in terms of frequencyor the frequency of beacons which recur in a known periodic manner dueto the communications channel structure. The communications channelframing structure defines, in some embodiments, such things as hopping,control or traffic channel segment definitions which may be prestored inthe mobile and accessed based on information derived from a receivedbeacon signal. Thus, the transmission of beacon signals in a periodicpredictable manner and use of a fixed communications channel structurewhich can be stored and associated with beacon information which can beused to determine which channel structure is being used, facilitatesinterpretation of resource assignments and determining when a particularsegment will occur at the new network access point before the mobileachieves symbol timing synchronization with the new network attachmentpoint, e.g., in the case of an inter-cell handoff.

With regard to the implementation of handoffs from the mobiles,perspective, in various embodiments, the handoff triggering mechanism isas described above in regard to the use of beacon signals to determinewhen a handoff should occur.

In some exemplary handoff embodiments, the mobile node uses a beaconsignal received from the handoff destination to determine one or more ofthe following: the cell ID, Sector ID and/or carrier frequency used bythe destination network point of attachment which transmitted thedetected beacon signal. The mobile node can also determine from thedetected beacon signal and, e.g., stored communications channelstructure information relating to different cell's and/or sectors, theframing structure of the communications channels used by the networkpoint of attachment to which the handoff is to be completed, e.g., asecond base station sector. From the determined framing structure andinformation about when the beacon signal was received, the mobile candetermine the time to drop the exiting wireless communications link andbegin establishing the new communications link. The timing may be basedin addition to the time the detected beacon signal was received,information about the time a dedicated resource, e.g., particular accesssegment, will occur at the destination network attachment point.

In many embodiments, the mobile node communicates its intent to handoffto a new network attachment point by sending one or more signals over anexisting communications link to the network attachment point, e.g., basestation sector, serving as the mobile node's current point of networkattachment. Thus, the mobile, in accordance with the invention, willnormally signal its intent to perform a handoff by sending a signal overthe existing wireless communications link. The current networkattachment point forwards the handoff signal from the mobile to thesecond network attachment point (handoff destination) and/or acts as aproxy for the mobile node and exchanges handoff signals with the secondnetwork attachment point on behalf of the mobile node. The destinationnetwork node assigns, e.g., dedicates, one or more air link resources tothe mobile node and communications information about the dedicatedresource(s) to the mobile node via the current network point ofattachment. The dedicated resources may include one or more sets oftones in an uplink communications channel, e.g., a portion of an accesssegment, to be used for, e.g., sending a registration signal, timingcontrol signal and/or power control signal to the base station as partof a registration operation The dedicated resources could also includeone or more device identifiers to be used when communicating with thenew network point of attachment, e.g., an on-state identifier to be usedwhen communicating in an on-state of operation and a hold=stateidentifier to be used when the mobile node operates in a hold state. Theresource information is communicated to the mobile node over the currentwireless communications link with the current point of networkattachment. The mobile node terminates, e.g., drops, the existingcommunications link before establishing the new communications link withthe network attachment point which is the destination of the handoff.The time at which the link is terminated may be based on the time of areceived beacon signal and expected time offset from the received beaconsignal to a communications segment or set of tones in a segment whichhas been dedicated to the mobile node for purposes of registering withthe new network point of attachment. The mobile node uses the resourcethat was dedicated by the new network attachment point to access the newnetwork point of attachment in a contention free manner and therebyestablish a communications link with the new network point ofattachment. Should something happen and the mobile node is unable tocomplete the establishment of the new communications link using thededicated resource, the mobile node in various embodiments will wait andattempt to perform the registration with the new point of networkattachment using contention based signaling.

Various embodiments using the general method described above but withminor variations intended to be particularly well suited for inter-cellor intra-cell applications are contemplated. In one particularinter-cell handoff embodiment, air link resources assigned by thenetwork access point to which the handoff is directed to, e.g., a secondbase station, includes a dedicated access segment in an uplink of thesecond base station. In particular exemplary example of such anembodiment the mobile determines the definition of the assigned accesssegment e.g., on what tone sets and at what OFDM symbol time it has beenassigned to use for registering, from the detected beacon signal and theinformation returned over the existing communications link via a firstbase station through which the mobile is coupled to the network at thetime the handoff is initiated. In such an exemplary embodiment, themobile drops the first link prior to the starting to transmit to thesecond base station using the assigned access segment. Using theassigned access segment, the mobile sends one or more signals to thebase station, e.g., registration, power control and/or timing controlsignals. In response to the signals sent on the uplink, the mobilereceives timing and/or power control signals from the second basestation and makes timing and/or power adjustments in response to thecontrol signals. In such an embodiment, as part of the registrationprocess, the mobile may get further dedicated resources such as ONidentifier, dedicated control channels to continue communications withthe second base station if they were not previously assigned. In variousembodiments of this type, the dedicated access segment is anon-contention bases access segment which and other mobile are notallowed to use the access segment that was dedicated to the mobile node.If use of the access segment is not successful, and a communicationslink is not established with the second base station, the mobile nodeattempts to register again through contention based signal competing forresources in the same manner that mobiles entering the system wouldregister without the benefit of previously allocated dedicatedregistration resources.

Some exemplary intra-cell handoff embodiments will now be discussed. Insome intra-cell handoff embodiments, e.g., inter-sector intra-celland/or inter-carrier intra-sector embodiments, the current and newnetwork access points are located in the same cell and are synchronizedin symbol timing and, optionally, carrier frequency. In such a handoffembodiment, at the start of the handoff the mobile is synchronized as aresult of the timing control performed with regard to the current linkwith the symbol timing of the current network point of attachment, e.g.,sector. In some of the intra-cell handoff embodiments, a dedicatedresource(s) assigned by the new network access point include dedicatedresources such as dedicated access segment, ON identifier for use whencommunicating with the new network attachment point, dedicated controlchannel segments to be used to perform timing and/or power controlcommunications with the new network access point. In some of theintra-cell handoff embodiments, the mobile does not have to transmit anyaccess, timing control and/or power control signal in order to set upthe second link and skips some or all of the signaling steps of thistype that would occur in the case of an inter-sector handoff. Given thatthe handoff is an intra-cell handoff, in many embodiments, messagesrelating to the handoff are localized between the network attachmentpoints within the cell and there is no need to use a backhaul linkbetween cells to complete the handoff. In some intra-cell handoffs, themobile drops the current communications link and within a very shortperiod of time, e.g., less than the time to complete an inter-cellhandoff, and in some cases in less than 5 milli-seconds, starts to usethe assigned dedicated channel resources dedicated by the new networkpoint of attachment and may, in fact, start transmitting user dataalmost immediately, e.g., without having to wait to first receive atiming and/or power control signal over the air from the new networkpoint of attachment.

FIG. 15 is a drawing 1500 illustrating an exemplary embodiment whichuses access segments in an uplink and downlink channel in accordancewith some embodiments of the present invention. While access and trafficsegments are shown as separate segments in terms of time in the downlinkin FIG. 15, it should be appreciated that the access and trafficsegments could be implemented using different sets of tones during thesame period of time. That is, while access and traffic segments may betime multiplexed in the downlink, time multiplexing is not necessarilyrequired for the invention.

Drawing 1500 includes a vertical axis 1502 representing uplink tones anda horizontal axis 1504 representing time. Each small division of thevertical axis 1502 represents a tone, while each small division of thehorizontal axis 1504 represents an OFDM symbol transmission interval.Row 1513 corresponds to the uplink channel while row 1543 corresponds tothe downlink channel. Beacons are periodically transmitted in the downlink channel in beacon slots 1541, 1541′. One or more beacon signals inthe beacon slots are transmitted at known fixed offsets from the accessslots 1514, 1414′ in the uplink channel. For example, at least onebeacon signal transmitted in slot 1541 will occur a fixed number ofsymbol transmission time periods from the start of uplink access slot1514′.

In addition to beacon slots the downlink includes access slots 1544,1544′ which can be used for communicating registration acknowledgmentsignals, WT timing control signals and/or WT power control signals toone or more wireless terminals, e.g., terminals seeking to establish acommunications link with the network attachment point with which uplinkand downlink channels 1513, 1543 are associated.

Uplink tones are shown and have been grouped into four exemplary sets oftones (1^(st) set of tones 1506, 2^(nd) set of tones 1508, 3^(rd) set oftones 1510, 4^(th) set of tones 1512). Downlink tones are not shown butmay also be grouped into sets for use by different wireless terminals.In the uplink channel 1513 OFDM symbol transmission time intervals aregrouped into exemplary intervals, e.g., slots, including access interval1514, traffic interval 1516, traffic interval 1518, and traffic interval1520. The sequence of intervals repeats over time as illustrated withaccess interval 1514′, traffic interval 1516′, traffic interval 1518′,traffic interval 1520′ and will have a known timing relationship to theintervals in the downlink channel 1543.

During an access interval WTs can use access segments, e.g., forregistration with a BS sector attachment point establishing a newwireless link. Two types of access segments are shown, contention basedaccess segments and dedicated access segments. Contention access segment1 1522 uses 1^(st) set of tones 1506 during access interval 1514;contention access segment 2 1524 uses 2^(nd) set of tones 1508 duringaccess interval 1514; dedicated access segment 1 1526 uses 3^(rd) set oftones 1510 during access interval 1514; dedicated access segment 2 1528uses 4^(th) set of tones 1512 during access interval 1514. Similarly,contention access segment 1 1522′ uses 1^(st) set of tones 1506 duringaccess interval 1514′; contention access segment 2 1524′ uses 2^(nd) setof tones 1508 during access interval 1514′; dedicated access segment 11526′ uses 3^(rd) set of tones 1510 during access interval 1514′;dedicated access segment 2 1528′ uses 4^(th) set of tones 1512 duringaccess interval 1514′.

During a traffic interval (1516, 1518, 1520, 1516′, 1518′, 1520′) thereare segments including uplink traffic channel segments in which WTs cansend user data to the base station sector attachment point via theestablished wireless link.

In some exemplary systems, when an exemplary WT is in an access state ofoperation, an access segment may be used for transmitting an uplinksignal including registration information, timing control information,and/or power control information. A registration signal sent in theuplink may be used for timing control and/or power control purposeswhile indicating an intent to register. Thus, a single uplink signal canserve multiple purposes. Alternatively, different signals can be usedfor each function. As part of the access operation the WT sends the BSsome uplink signal using an access segment from which the base stationperforms measurements. The WT will use a dedicated, contention freeslot, for transmission of the uplink signal assuming it was assigned adedicated segment for this purpose. The uplink signal may be, e.g., asignal that is transmitted at a predetermined WT power level that istransmitted, e.g., at a predetermined time within the access segmentwith respect to the WTs current timing settings. The base stationreceives the uplink signal, performs measurements, and sends back to thewireless terminal a signal or signals, e.g., timing and/or power controlsignals, on a downlink channel segment or segments, e.g., downlinksegments in a corresponding downlink access slot 1546 or 1546′. Inaddition to timing and/or power control signals, the BS may send anacknowledgment of the uplink registration signal. The WT performs anycommanded adjustments and can then send uplink signals including userdata on allocated uplink traffic channel segments, e.g., trafficsegments 1516, 1518, 1520, etc.

In accordance with some embodiments of the invention, a dedicated accesssegment, e.g., dedicated access segment 1 1526, is assigned to awireless terminal which has requested a handoff. The assignmentinformation of the dedicated access segment is conveyed to the WT viathe current wireless link.

In general, the use of dedicated access segments in handoff operationsprovides a more efficient handoff. Benefits of using dedicated air linkresources assigned via an exiting communications link and timinginformation obtained from a broadcast signal transmitted over the airinto the frequency band used by the WT, may include less time lostbetween the termination of the original wireless link and theestablishment of the new wireless link, a higher handoff success ratedue to the use of a dedicated access segment as opposed to the usualcontention based (collision prone) access segments, and/or eliminationof some operations such as some timing control adjustments, e.g., in anintra-cell handoff.

The contention access segments, e.g., contention segment 1 1522′, isused by a WT which does not currently have an established wireless link,e.g., a WT which has just powered on, to register with a BS sectorattachment point and establish a wireless link. In some embodiments, ifa handoff fails while using a dedicated access segment, the WT thenseeks to register using a contention based access segment. Use ofcontention access segments may result in a collision with another WTseeking to establish a wireless link, resulting in an unsuccessfulregistration attempt. When using a contention based access segment, theWT generates and transmits an uplink signal to the BS sector attachmentpoint which is received, measured, and used by the BS to calculate WTtiming control and power control adjustment information which is sent tothe WT via downlink segments. Thus, when the contention based accessoperation is performed, timing control is normally performed prior tothe WT sending user data in the uplink.

In some cases, where the handoff request is an intra-cell handoffrequest, e.g., inter-sector or intra-sector inter-carrier, registrationsignals sent in the access segment are not used for timing controloperations and/or registration via an access segment may be skippedentirely. Assuming that the sectors are timing synchronized and the WThas been assigned, via the exiting link, the dedicated resourcesrequired to send user data, a WT can begin transmitting user data and/orother signals to the new network attachment point which assigned thenecessary resources without having to first perform registration, timingcontrol and/or power control via the air link with the new point ofattachment. This is possible since, in some embodiments, timingsynchronization is maintained across sectors of a cell.

FIG. 16 illustrates an exemplary wireless communications system 1600including three exemplary cells (cell 1 1602, cell 2 1604, cell 3 1606),each cell indicated by a solid line circle. Each cell (1602, 1604, 1606)represents the wireless coverage area for a base station (1608, 1610,1612) located at the center of the cell (1602, 1604, 1606),respectively. Each cell (1602, 1604, 1606) is subdivided into threesectors A, B, and C. Cell 1 1602 includes sector A 1614, sector B 1616,and sector C 1618. Cell 2 1604 includes sector A 1620, sector B 1622,and sector C 1624. Cell 3 1606 includes sector A 1626, sector B 1628,and sector C 1630. Carrier f₁ is indicated by a dotted line as shown inlegend 1632; carrier f₂ is indicated by a dot/dash line as shown inlegend 1634; carrier f₃ is indicated by a dash line as shown in legend1636. Each carrier frequency f₁, f₂, f₃ is associated with a 1.25 MHzbandwidth segment of the 5 MHz available total BW in the exemplaryembodiment, and the BW segments are non-overlapping. The radius of each(dotted, dot/dash, or dashed) line is indicative of the transmitterpower associated with the carrier in the given sector. In FIG. 16, thereis a frequency reuse factor of 1, i.e., the same set of frequencies isused in each sector and in each cell.

In each of the three cells (1602, 1604, 1606), the base stations sectorA transmitter uses carrier frequency (f₁, f₂, f₃) at a (high,intermediate, low) power level, respectively, for communications, e.g.,downlink traffic and control channel signals, from the base station(1608, 1610, 1612) to wireless terminals. In each cell (1602, 1604,1606), the base station sector B transmitter uses carrier frequency (f₂,f₃, f₁) at a (high, intermediate, low) power level, respectively,communications, e.g., downlink traffic and control channel signals, fromthe base station (1008, 1010, 1012) to wireless terminals; the basestation sector C transmitter uses carrier frequency (f₃, f₁, f₂) at a(high, intermediate, low) power level, respectively, for communications,e.g., downlink traffic and control channel signals, from the basestation (1608, 1610, 1612) to wireless terminals. The following notationis used to describe the base station transmitter power levels in system1600 with respect to the carrier frequencies: (cell, sector, high powercarrier/intermediate power carrier/low power carrier): (cell referencenumber, sector reference number, arc line reference number for highpower carrier/arc line reference number for intermediate powercarrier/arc line reference number for low power carrier). System 1600includes: (cell 1, sector A, f₁/f₂/f₃):(1602, 1614, 1638/1640/1642);(cell 1, sector B, f₂/f₃/f₁):(1602, 1616, 1644/1646/1648); (cell 1,sector C, f₃/f₁/f₂):(1602, 1618, 1650/1652/1654); (cell 2, sector A,f₁/f₂/f₃):(1604, 1620, 1656/1658/1660); (cell 2, sector B,f₂/f₃/f₁):(1604, 1622, 1662/1664/1666); (cell 2, sector C,f₃/f₁/f₂):(1604, 1624, 1668/1670/1672); (cell 3, sector A,f₁/f₂/f₃):(1606, 1626, 1674/1676/1678); (cell 3, sector B,f₂/f₃/f₁):(1606, 1628, 1680/1682/1684); (cell 3, sector C,f₃/f₁/f₂):(1606, 1630, 1686/1688/1690).

FIG. 16 represents the same level of frequency reuse throughout eachsector of a system and may represent a system in an advanced level ofdeployment, e.g., where a deployment program has been completed and/orwhere the service provider has a larger customer base with high demandswhich can justify such a deployment level.

While the three carriers are transmitted at different power levels P₁,P₂ P₃, in each sector. In various embodiments there is a fixedrelationship between the three power levels P₁, P₂ P₃, that is used ineach sector. In one such embodiment P₁>P₂>P₃ in each sector and theratio of P₁ to P₂ and P₂ to P₃ is the same regardless of the sector.Uplink carriers may be associated with each of the downlink carriers.

Inter-cell, inter-sector, and intra-sector inter-carrier handoffs, inaccordance with the methods of the present invention may occur with thesystem of FIG. 16.

In implementations such as the one shown in FIG. 16, each carrier andsector has associated with it one or more modules that can be used as anetwork attachment point by a mobile node. Switching between carrierswithin a cell results in switching between network attachment points andthus a handoff between network attachment points within the cell. In thecase of sectors which support multiple carriers this may involve ahandoff from a network attachment point corresponding to a first carrierto a network attachment point corresponding to a second, differentcarrier, within the same sector.

FIG. 17 illustrates a base station sector 1701 with two exemplarynetwork attachment points 1801, 1807 which correspond to differentcarriers f₀ and f₂ respectively. In the case of some embodiments of theFIG. 16 example, each sector would include three network attachmentpoint modules, e.g., the sector 1701 would include a third networkattachment point corresponding to carrier f₁. Thus, there would be athird network attachment point module in addition to attachment pointmodules 1801, 1807 shown in FIG. 17.

Each network attachment point 1801, 1807 can serve as a wirelessterminal's attachment point, via a wireless connection, to the networkto which the base station including sector 1701 is coupled. While shownas being in a sector, it can be appreciated that network attachmentpoints 1801, 1807 could be in different sectors of the same cell or insectors of different cells rather than in the sector of the same cell.Each of the network attachment points 1801, 1807 uses a differentfrequency band 1718, 1722 for communicating user data. Networkattachment point module 1 1801 includes a first BS transmitter 1702 m afirst BS sector receiver 1703 and a first control module 1713 which arecoupled together by a bus. The control module 1713 causes the firstnetwork attachment point to operate in accordance with the invention,e.g., in the manner described above, and interacts with other networkattachment points to coordinate handoffs and assign air link resources.The second network attachment point module 1807 includes a second basestation sector transmitter 1704, a corresponding BS sector receiver 1705and a second control module 1715 which are coupled together by a bus.Control module 1715 causes the second network attachment point tooperate in accordance with the invention, e.g., in the manner describedabove, and interacts with other network attachment points to coordinatehandoffs and assign air link resources. The control modules of thedifferent network access links are coupled to control modules of othersectors by links within the BS in which they are included and to networkattachment point control modules of other cells by a back haul linkimplemented, e.g., with fiber optic or wire connections.

Use of first and second transmitters 1702, 1704, corresponding todifferent network attachment points, will now be described. Transmitters1702, 1704 transmit downlink signals including, e.g., ordinary trafficchannel signals, e.g., user data, optionally pilot signals, and beaconsignals. The relative timing of the various signals may be as shown inFIG. 15. The transmitters 1702, 1704 may use different antennas directedtowards different sectors or cells. Signaling from each sectortransmitter includes ordinary signaling, e.g., assignment signals,optionally pilot signals, and/or optionally beacon signals, in its owndesignated carrier frequency band and beacon signals in one or more,e.g., the other two, carrier frequency bands used in a cell. The firsttransmitter 1702 transmits downlink signals 1706 including, e.g.,Transmitter 1 downlink traffic signals, Transmitter 1 assignmentsignals, Transmitter 1 WT control signals, optionally Transmitter 1pilot signals, and/or Transmitter 1 beacon signals into a frequency band1718 corresponding to carrier frequency f₀ 1724, Transmitter 1 beaconsignals 1708 into a frequency band 1720 corresponding to carrierfrequency f₁ 1726, and Transmitter 1 beacon signals 1710 into afrequency band 1722 corresponding to carrier frequency f₂ 1728.Transmitter 2 1704 transmits downlink signals 1712 including, e.g.,Transmitter 2 downlink traffic signals, Transmitter 2 assignmentsignals, optionally Transmitter 2 pilot signals, Transmitter 2 WTcontrol signals and/or Transmitter 2 beacon signals into frequency band1722 corresponding to carrier frequency f₂ 1728. Transmitter 2 1704 alsotransmits Transmitter 2 beacon signals into frequency band 1718corresponding to carrier frequency f₀ 1724, and Transmitter 2 beaconsignals 1716 into frequency band 1720 corresponding to carrier frequencyf₁ 1726.

Assume that a WT 1730 is tuned to carrier frequency band 1718 withcarrier frequency f₀ 1724. The receiver in the WT 1730 receives twosignal components 1732, 1734, the first signal component 1732, includinge.g., ordinary signaling, assignment signals, pilot signals, and/orbeacon signals from the Transmitter 1 602 which are processed. At thesame or a different time, second signal component 1734, which includes,e.g., the beacon signal from the second Transmitter 2 1704 is receivedand processed. Based on the energy in the beacon signals received fromthe transmitters (1702, 1704) corresponding to the different carriers f₀and f₂, respectively, the WT may initiate a handoff from the firstnetwork attachment point 1801 to the second network attachment point1807 using the existing communication link with network attachment point1801. Thus, in accordance with the invention, the WT 1730 can requestand receive dedicated air link resources form network attachment point1807 via the existing communications link and then terminate the linkand establish a new link with attachment point 1807, e.g., at a timedetermined from a beacon signal received from transmitter 1704 andassignment information communicated over the link with the first networkattachment point 1801 prior to termination of the link.

While described in the context of an OFDM system, the methods andapparatus of the present invention are applicable to a wide range ofcommunications systems including many non-OFDM and/or non-cellularsystems.

In various embodiments nodes described herein are implemented using oneor more modules to perform the steps corresponding to one or moremethods of the present invention, for example, signal processing, beacongeneration, beacon ID, beacon measuring, beacon comparison, handoff,message generation and/or transmission steps. In some embodimentsvarious features of the present invention are implemented using modules.Such modules may be implemented using software, hardware or acombination of software and hardware. Many of the above describedmethods or method steps can be implemented using machine executableinstructions, such as software, included in a machine readable mediumsuch as a memory device, e.g., RAM, floppy disk, etc. to control amachine, e.g., general purpose computer with or without additionalhardware, to implement all or portions of the above described methods,e.g., in one or more nodes. Accordingly, among other things, the presentinvention is directed to a machine-readable medium including machineexecutable instructions for causing a machine, e.g., processor andassociated hardware, to perform one or more of the steps of theabove-described method(s).

Numerous additional variations on the methods and apparatus of thepresent invention described above will be apparent to those skilled inthe art in view of the above description of the invention. Suchvariations are to be considered within the scope of the invention. Themethods and apparatus of the present invention may be, and in variousembodiments are, used with CDMA, orthogonal frequency divisionmultiplexing (OFDM), and/or various other types of communicationstechniques which may be used to provide wireless communications linksbetween access nodes and mobile nodes. In some embodiments the accessnodes are implemented as base stations which establish communicationslinks with mobile nodes using OFDM and/or CDMA. In various embodimentsthe mobile nodes are implemented as notebook computers, personal dataassistants (PDAs), or other portable devices includingreceiver/transmitter circuits and logic and/or routines, forimplementing the methods of the present invention.

1. A network attachment point for use in a multi-carrier communicationsystem comprised of at least one wireless terminal in an activecommunication session with a serving network attachment point, where theactive communication is in a first frequency band which is differentfrom a second frequency band associated with the network attachmentpoint for transmitting user data, comprising: means for generating abroadcast signal message having a predetermined frequency into the firstfrequency band on a periodic basis; and means, responsive to a requestfrom a wireless terminal received from the corresponding serving networkattachment point, for communicating dedicated system handoff informationto the wireless terminal over the active communication session with theserving network attachment point without the wireless terminalestablishing a communication session with the network attachment point.2. The network attachment point of claim 1, further comprising means fordetecting a request from the serving network attachment point toinitiate a secure communication link over a network therewith.
 3. Thenetwork attachment point of claim 2, further comprising means forassigning at least one of on-state identifier and hold-state identifierto wireless terminal, means for reserving resources for the wirelessterminal, and means for transmitting information, including access stateinformation in which the network attachment point has reserved dedicatedtone set-up link communication segment for registration by the wirelessterminal, this information being communicated to the wireless terminalvia the secure communication link with the serving network attachmentpoint.
 4. The network attachment point of claim 3, further comprising,after termination by the wireless terminal of the active communicationsession with the serving network attachment point, means for detecting awireless terminal registration attempt over the dedicated tone set-uplink communication segment in a specified access slot.
 5. The networkattachment point of claim 4, further comprising means for registeringthe wireless terminal with the network, including means for transmittingat least one of timing synchronization and power control signaling tothe wireless terminal.
 6. The network attachment point of claim 5,further comprising means for detecting successful registration and meansfor releasing assigned identifiers and resources to establish an activecommunications session with the wireless terminal.
 7. The networkattachment point of claim 6, wherein the broadcast system message is abeacon signal having known timing offsets from uplink channel segmentsfor use in a dedicated fashion by the wireless terminal during handoffinitiation.
 8. The network attachment point of claim 7, wherein themulti-carrier communication system is an OFDM system, the wirelessterminal is a mobile node, and the network attachment point and theserving network attachment point are base stations.
 9. The networkattachment point of claim 8, wherein the network attachment point andthe serving network attachment points are located in a same cell. 10.The network attachment point of claim 1, wherein the multi-carriercommunication system is an OFDM system, the wireless terminal is amobile node, and the network attachment point and the serving networkattachment point are base stations.
 11. The network attachment point ofclaim 10, wherein the network attachment point and the serving networkattachment points are located in a same cell.
 12. A network attachmentpoint for use in a multi-carrier communication system comprised of atleast one wireless terminal configured to generate requests to initiatehandoff from the network attachment point to an adjacent networkattachment point, where the adjacent network attachment point usesdifferent frequency bands to establish communication sessions with theat least one wireless terminal than the frequency band used by thenetwork attachment point, and where the adjacent network attachmentpoint generates broadcast system messages in the frequency band of thenetwork attachment point detectable by a wireless terminal when in anactive communication session with the network attachment point tofacilitate system handoff, the network attachment point comprising:means for receiving a request from the wireless terminal to initiatehandoff communications between the wireless terminal and the adjacentnetwork attachment point; and means for exchanging dedicated systemhandoff information between the adjacent network attachment point andthe wireless terminal over the active communication session with thenetwork attachment point without the wireless terminal establishing acommunication session with the adjacent network attachment point. 13.The network attachment point of claim 12, wherein the means forexchanging dedicated system handoff information includes means forreceiving from the adjacent network attachment point and means forpassing to the wireless terminal over a currently active communicationlink between the wireless terminal and the network attachment point,information identifying registration operations to be performed by thewireless terminal after terminating the currently active communicationlink.
 14. The network attachment point of claim 13, wherein thebroadcast system message is a beacon signal.
 15. The network attachmentpoint of claim 12, wherein the broadcast system message is a beaconsignal.
 16. The network attachment point of claim 15, wherein the beaconsignal includes at least one of a cell identifier and a sectoridentifier, the means for receiving a request including the at least oneof a cell identifier and a sector identifier.
 17. The network attachmentpoint of claim 16, wherein the dedicated system handoff information isdedicated airlink resource assignment information associated with theadjacent network attachment point for use by the wireless terminal whenperforming a system handoff, the information being received in the formof a packet from the adjacent network attachment point to the wirelessterminal over a currently active communication link between the wirelessterminal and the network attachment point.
 18. The network attachmentpoint of claim 17, wherein the dedicated airlink resource assignmentinformation comprises information for the wireless terminal tocommunicate with the adjacent network attachment point over a dedicatedresource assigned to the wireless terminal, this resource including aset of tones in an uplink communications segment, the wireless terminalbeing adapted to identify the set of tones from information receivedfrom the adjacent network attachment point via the uplink communicationssegment and from the information regarding the structure of thecommunications segment derived from the beacon signal.
 19. The networkattachment point of claim 18, wherein the multi-carrier communicationsystem is an OFDM system, the wireless terminal is a mobile node, andthe network attachment point and the adjacent network attachment pointare base stations.
 20. The network attachment point of claim 12, whereinthe multi-carrier communication system is an OFDM system, the wirelessterminal is a mobile node, and the network attachment point and theadjacent network attachment point are base stations.
 21. The networkattachment point of claim 20, further comprising means for terminating acurrently active communication link between the wireless terminal inresponse to a request to do so by the wireless terminal which thenestablishes a new communication link with the adjacent networkattachment point, the new communication link being established onlyafter the currently active communication link is terminated and on thebasis of system handoff information received from the adjacent networkattachment point via the network attachment point.
 22. The networkattachment point of claim 12, wherein the first and adjacent networkattachment points are located in a same cell.
 23. The network attachmentpoint of claim 12, wherein the means for exchanging dedicated systemhandoff information includes: means for receiving from the wirelessterminal a handoff request to the adjacent network attachment point viaa currently active communication link with the first network attachmentpoint and communicating the request directly to the adjacent networkattachment point over a separate communication link; and means forreceiving from the adjacent network attachment point, and communicatingback to the wireless terminal via the currently active communicationlink, assigned dedicated allocated resources including at least one ofan identifier and dedicated uplink segments.
 24. The network attachmentpoint of claim 23, wherein the broadcast system message is a beaconsignal, the wireless terminal determining a timing associated with theassigned dedicated allocated resources based on the beacon signal andpredefined timing relationships thereof to the dedicated uplinksegments.
 25. The network attachment point of claim 24, furthercomprising means for terminating the currently active communication linkprior to a first assigned dedicated allocated resource occurrence to thewireless terminal by the adjacent network attachment point.
 26. Thenetwork attachment point of claim 25, wherein the request from thewireless terminal for a handoff request wireless terminal is based onthe wireless terminal detecting a weak signal strength condition withrespect to the currently active communications link with the networkattachment point.
 27. A method of operating a network attachment pointfor use in a multi-carrier communication system comprised of at leastone wireless terminal in an active communication session with a servingnetwork attachment point, where the active communication is in a firstfrequency band which is different from a second frequency bandassociated with the network attachment point for transmitting user data,comprising: generating a broadcast signal message having a predeterminedfrequency into the first frequency band on a periodic basis; andtransmitting, responsive to a request from a wireless terminal receivedfrom the corresponding serving network attachment point, dedicatedsystem handoff information to the wireless terminal over the activecommunication session with the serving network attachment point withoutthe wireless terminal establishing a communication session with thenetwork attachment point.
 28. The method of claim 27, further comprisingdetecting a request from the serving network attachment point toinitiate a secure communication link over a network therewith.
 29. Themethod of claim 28, further comprising assigning at least one ofon-state identifier and hold-state identifier to wireless terminal,reserving resources for the wireless terminal, and transmittinginformation, including access state information in which the networkattachment point has reserved dedicated tone set-up link communicationsegment for registration by the wireless terminal, this informationbeing communicated to the wireless terminal via the secure communicationlink with the serving network attachment point.
 30. The method of claim29, further comprising, after termination by the wireless terminal ofthe active communication session with the serving network attachmentpoint, detecting a wireless terminal registration attempt over thededicated tone set-up link communication segment in a specified accessslot.
 31. The method of claim 30, further comprising registering thewireless terminal with the network, including transmitting at least oneof timing synchronization and power control signaling to the wirelessterminal.
 32. The method of claim 31, further comprising detectingsuccessful registration and releasing assigned identifiers and resourcesto establish an active communications session with the wirelessterminal.
 33. The method of claim 32, wherein the broadcast systemmessage is a beacon signal having known timing offsets from uplinkchannel segments for use in a dedicated fashion by the wireless terminalduring handoff initiation.
 34. The method of claim 33, wherein themulti-carrier communication system is an OFDM system, the wirelessterminal is a mobile node, and the network attachment point and theserving network attachment point are base stations.
 35. The method ofclaim 34, wherein the network attachment point and the serving networkattachment points are located in a same cell.
 36. The method of claim27, wherein the multi-carrier communication system is an OFDM system,the wireless terminal is a mobile node, and the network attachment pointand the serving network attachment point are base stations.
 37. Themethod of claim 36, wherein the network attachment point and the servingnetwork attachment points are located in a same cell.
 38. A method ofoperating a network attachment point for use in a multi-carriercommunication system comprised of at least one wireless terminalconfigured to generate requests to initiate handoff from the networkattachment point to an adjacent network attachment point, where theadjacent network attachment point uses different frequency bands toestablish communication sessions with the at least one wireless terminalthan the frequency band used by the network attachment point, and wherethe adjacent network attachment point generates broadcast systemmessages in the frequency band of the network attachment pointdetectable by a wireless terminal when in an active communicationsession with the network attachment point to facilitate system handoff,the method comprising: in response to a request from the wirelessterminal to initiate handoff communications between the wirelessterminal and the adjacent network attachment point, exchanging dedicatedsystem handoff information between the adjacent network attachment pointand the wireless terminal over the active communication session with thenetwork attachment point without the wireless terminal establishing acommunication session with the adjacent network attachment point. 39.The method of claim 38, wherein the exchanging dedicated system handoffinformation includes receiving from the adjacent network attachmentpoint and passing to the wireless terminal over a currently activecommunication link between the wireless terminal and the networkattachment point, information identifying registration operations to beperformed by the wireless terminal after terminating the currentlyactive communication link.
 40. The method of claim 39, wherein thebroadcast system message is a beacon signal.
 41. The method of claim 38,wherein the broadcast system message is a beacon signal.
 42. The methodof claim 41, wherein the beacon signal includes at least one of a cellidentifier and a sector identifier, the receiving a request includingthe at least one of a cell identifier and a sector identifier.
 43. Themethod of claim 42, wherein the dedicated system handoff information isdedicated airlink resource assignment information associated with theadjacent network attachment point for use by the wireless terminal whenperforming a system handoff, the information being received in the formof a packet from the adjacent network attachment point to the wirelessterminal over a currently active communication link between the wirelessterminal and the network attachment point.
 44. The method of claim 43,wherein the dedicated airlink resource assignment information comprisesinformation for the wireless terminal to communicate with the adjacentnetwork attachment point over a dedicated resource assigned to thewireless terminal, this resource including a set of tones in an uplinkcommunications segment, the wireless terminal being adapted to identifythe set of tones from information received from the adjacent networkattachment point via the uplink communications segment and from theinformation regarding the structure of the communications segmentderived from the beacon signal.
 45. The method of claim 44, wherein themulti-carrier communication system is an OFDM system, the wirelessterminal is a mobile node, and the network attachment point and theadjacent network attachment point are base stations.
 46. The method ofclaim 38, wherein the multi-carrier communication system is an OFDMsystem, the wireless terminal is a mobile node, and the networkattachment point and the adjacent network attachment point are basestations.
 47. The method of claim 46, further comprising terminating acurrently active communication link between the wireless terminal inresponse to a request to do so by the wireless terminal which thenestablishes a new communication link with the adjacent networkattachment point, the new communication link being established onlyafter the currently active communication link is terminated and on thebasis of system handoff information received from the adjacent networkattachment point via the network attachment point.
 48. The method ofclaim 38, wherein the first and adjacent network attachment points arelocated in a same cell.
 49. The method of claim 38, wherein theexchanging dedicated system handoff information includes: receiving fromthe wireless terminal a handoff request to the adjacent networkattachment point via a currently active communication link with thefirst network attachment point and communicating the request directly tothe adjacent network attachment point over a separate communicationlink; and receiving from the adjacent network attachment point, andcommunicating back to the wireless terminal via the currently activecommunication link, assigned dedicated allocated resources including atleast one of an identifier and dedicated uplink segments.
 50. The methodof claim 49, wherein the broadcast system message is a beacon signal,the wireless terminal determining a timing associated with the assigneddedicated allocated resources based on the beacon signal and predefinedtiming relationships thereof to the dedicated uplink segments.
 51. Themethod of claim 50, further comprising terminating the currently activecommunication link prior to a first assigned dedicated allocatedresource occurrence to the wireless terminal by the adjacent networkattachment point.
 52. The method of claim 51, wherein the request fromthe wireless terminal for a handoff is based on the wireless terminaldetecting a weak signal strength condition with respect to the currentlyactive communications link with the network attachment point.
 53. Acomputer program product for use in a base station to be employed in amulti-carrier communication system comprised of at least one mobiledevice in an active communication session with a serving base station,where the active communication is in a first frequency band which isdifferent from a second frequency band associated with the base stationfor transmitting user data, the computer program product including acomputer-readable medium having instructions to, generate a beaconsignal having a predetermined frequency into the first frequency band ona periodic basis; and transmit, responsive to a request from a wirelessterminal received from the corresponding serving base station, dedicatedsystem handoff information to the mobile device over the activecommunication session with the serving base station without the mobiledevice establishing a communication session with the base station. 54.The computer program product of claim 53, further comprising aninstruction to request from the serving base station to initiate asecure communication link over a network.
 55. The computer programproduct of claim 54, further comprising instructions to assign at leastone of on-state identifier and hold-state identifier to the mobiledevice, reserve resources for the mobile device, and transmitinformation, including access state information in which the basestation has reserved dedicated tone set-up link communication segmentfor registration by the mobile device, this information beingcommunicated to the mobile device via the secure communication link withthe serving base station.
 56. The computer program product of claim 55,further comprising, after termination by the mobile device of the activecommunication session with the serving base station, an instruction todetect a mobile device registration attempt over the dedicated toneset-up link communication segment in a specified access slot.
 57. Thecomputer program product of claim 53, further comprising an instructionto register the mobile device with the network, including an instructionto transmit at least one of timing synchronization and power controlsignaling to the mobile device.
 58. The computer program product ofclaim 57, further comprising instructions to detect successfulregistration and release assigned identifiers and resources to establishan active communications session with the mobile device.
 59. Thecomputer program product of claim 58, wherein the beacon signal hasknown timing offsets from uplink channel segments for use in a dedicatedfashion by the mobile device during handoff initiation.
 60. A computerprogram product for use in a base station to be employed in amulti-carrier communication system comprised of at least one mobiledevice configured to generate requests to initiate handoff from the basestation to an adjacent base station, where the adjacent base stationuses different frequency bands to establish communication sessions withthe at least one mobile device than the frequency band used by the basestation, and where the adjacent base station generates a beacon signalin the frequency band of the base station detectable by a mobile devicewhen in an active communication session with the base station tofacilitate system handoff, the computer program product including acomputer-readable medium having instructions to, in response to arequest from the mobile device to initiate handoff communicationsbetween the mobile device and the adjacent base station, exchangededicated system handoff information between the adjacent base stationand the mobile device over the active communication session with thebase station without the mobile device establishing a communicationsession with the adjacent base station.
 61. The computer program productof claim 60, wherein the instruction to exchange dedicated systemhandoff information includes instructions to receive from the adjacentbase station and pass to the mobile device over a currently activecommunication link between the mobile device and the network basestation, information identifying registration operations to be performedby the mobile device after terminating the currently activecommunication link.
 62. The computer program product of claim 61,wherein the beacon signal includes at least one of a cell identifier anda sector identifier, the request to initiate handoff including the atleast one of a cell identifier and a sector identifier.
 63. The computerprogram product of claim 62, wherein the dedicated system handoffinformation is dedicated airlink resource assignment informationassociated with the adjacent base station for use by the mobile devicewhen performing a system handoff, the information being received in theform of a packet from the adjacent base station to the mobile deviceover a currently active communication link between the mobile device andthe base station.
 64. The computer program product of claim 63, whereinthe dedicated airlink resource assignment information comprisesinformation for the mobile device to communicate with the adjacent basestation over a dedicated resource assigned to the mobile device, thisresource including a set of tones in an uplink communications segment,the mobile device being adapted to identify the set of tones frominformation received from the adjacent base station via the uplinkcommunications segment and from the information regarding the structureof the communications segment derived from the beacon signal.
 65. Thecomputer program product of claim 64, further comprising instructions toterminate a currently active communication link between the mobiledevice in response to a request to do so by the mobile device which thenestablishes a new communication link with the adjacent base station, thenew communication link being established only after the currently activecommunication link is terminated and on the basis of system handoffinformation received from the adjacent base station via the basestation.
 66. The computer program product of claim 65, wherein the basestation and adjacent base station are located in a same cell.
 67. Thecomputer program product of claim 60, wherein the instruction toexchange dedicated system handoff information includes instructions to:receive from the mobile device a handoff request to the adjacent basestation via a currently active communication link with the base stationand communicating the request directly to the adjacent base station overa separate communication link; and receive from the adjacent basestation, and communicating back to the mobile device via the currentlyactive communication link, assigned dedicated allocated resourcesincluding at least one of an identifier and dedicated uplink segments.68. The computer program product int of claim 67, wherein the mobiledevice determines a timing associated with the assigned dedicatedallocated resources based on the beacon signal and predefined timingrelationships thereof to the dedicated uplink segments, the computerprogram product further comprising instructions to terminate thecurrently active communication link prior to a first assigned dedicatedallocated resource occurrence to the mobile device by the adjacent basestation.